Popular Science Monthly/Volume 10/November 1876/Nature of the Invertebrate Brain II
IT now remains for us to consider the disposition of the nervous system in some of the principal types of the sub-kingdom Mollusca.
These are animals wholly different in kind from those we have just been considering, mostly aquatic, and all of them devoid of hollow, articulated, locomotor appendages. Their organs of vegetative life attain a disproportionate development. On the other hand, what are termed the "organs of relation" present a wide range of variation, as may be imagined from the fact that while some of the simplest representatives of the Mollusca consist of mere motionless sacs or bags, containing organs of digestion, respiration, circulation, and generation, its more complex forms are active predatory creatures, endowed with remarkable and varied powers of locomotion, and with sense-organs as keen and as highly developed as those of insects. The lower type is represented by the motionless ascidian, and the higher by the active and highly-endowed cuttle-fish.
Omitting any reference to the Polyzoa, we may turn our attention first of all to the Tunicata, of which the solitary ascidians may be taken as the type. They are marine animals, possessing no powers of locomotion, and having no head. The current of sea-water, serving for respiratory purposes, and, at the same time, containing food-particles, enters a large branchial chamber, through an open, funnel-like projection of the investing tunic of the animal, the orifice of which is guarded by sensitive tentacula and a sphincter muscle. The mouth is situated at the bottom of this branchial sac, down the side of which minute particles of food are swept by ciliary action, so as to be brought within the simple commencement of the œsophagus. The effete sea-water passes through the walls of this branchial cavity into a general body-chamber, in which the viscera are contained. This cavity is bounded externally by a muscular expansion, lining the outer cellulose tunic. By the periodical contraction of this muscular sac, the water which enters it, together with food-residues and ova, is expelled through another funnel-like opening, adjacent to and very similar to that by which it gains entry to the branchial chamber.
Although these ascidians have a definite alimentary canal, a circulatory system, and respiratory organs, together with a distinct genital apparatus, their life of relation with the external world is of the simplest description. They are stationary creatures, and have no prehensile organs, food being brought to the commencement of their alimentary canal by ciliary action.
In correspondence with such a simple mode of life, we might expect to find a very rudimentary nervous system, and this expectation is fully realized. The Tunicata possess a single small nervous ganglion lying between the bases of the two funnels through which water is taken in and discharged. This ganglion receives branches from the tentacula guarding the orifice of the oral funnel, and possibly from the branchial chamber, while it gives off outgoing, filaments to the various parts of the muscular sac, and perhaps to the alimentary canal, and some of the other internal organs. In some of the solitary Tunicata a rudimentary visual function is presumed to exist. At all events, pigment-spots are situated on, or in very close relation with, the solitary ganglion. This single body seems to serve for the performance, in a rudimentary manner, of the various functions discharged by at least two pairs of ganglia in a large number of higher Mollusca, viz., those known as the cerebral and the parieto-splanchnic or branchial.
The brachiopods are among the oldest and most wide-spread of the forms of life in the fossil state, and the geographical distribution of their living representatives at the present day is also very wide. Like the Tunicata, they are headless organisms, and lead a sedentary existence, attached either by a pedicle or by one division of their bivalve shells. The mouth is unprovided with any appendages for grasping food—nutritive particles being brought to it by means of ciliary currents. Numerous muscles exist which connect the valves of the shell to one another, and with the inclosed animal. And, though the visceral organization of the brachiopods is somewhat complex, no definite sense-organs have yet been detected in any of them. In the nervous system of these sedentary animals, there is, therefore, nothing answering to a brain as it is ordinarily constituted, though ganglia exist around the œsophagus which must receive afferent impressions of some kind, and from which branches proceed to the various muscles and viscera of the body.
Such low sensory endowments as are presented by the Brachiopoda would be wholly incompatible with that degree of visceral complexity of organization which they possess, had it not been for the fact that they lead such a passive existence in respect to quest of food. They do not go in search of it at all—they remain securely anchored while food is brought to the entrance of their alimentary canal by means of cilia. The absence of sense-organs and of a brain is, indeed, only compatible with a quasi-vegetative existence such as this.
The lamellibranchs, or ordinary headless bivalve Mollusca, also include some representatives—such as the oyster and its allies—which lead a sedentary life after the fashion of the Mollusca already mentioned. The valves of the shell in these lamellibranchs are lateral, instead of being dorsal and ventral as among the branchiopods. The shell is, however, closed by a single adductor muscle, and it is opened, when this relaxes, by means of an elastic hinge.
The mouth of the oyster is surrounded by four labial appendages, whose functions are not very definitely known. It presents no other appendages of any kind in the neighborhood of the mouth, and, as in the two types of Mollusca already described, the food which it swallows is brought to the entrance of its œsophagus by means of ciliary currents. This well-known animal has a large and important nervous ganglion (Fig. 8, b) situated posteriorly, and close to the great adductor muscle. It gives off' branches to this muscle, to each half of the mantle, to the gills (c, c), and it sends forward two long parallel branches (d, d), which serve to connect it with a much smaller anterior ganglion (a, a) situated on each side of the mouth. These anterior or labial ganglia are joined by a commissure arching over the mouth, and also by a more slender thread beneath the mouth, from which filaments (e) are given off to the stomach. These latter filaments may be considered to have a function similar to that of the stomatogastric nerves in insects. The anterior ganglia receive nerves (f) from the labial processes, probably for the most part afferent in function. At all events, these processes have no distinct muscular structure.
Other lamellibranchs possess a remarkable muscular appendage known as the foot, which is in relation with an additional single or double nervous ganglion, and is used in various ways as an organ of locomotion. The animals possessing this organ are also provided with a second adductor muscle for closing their shells. Speaking of the various uses of the foot among bivalves, Prof. Owen says: "To some which rise to the surface of the water it acts, by its expansion, as a float; to others it serves by its bent form as an instrument to drag them along the sands; to a third family it is a burrowing organ; to many it aids in the execution of short leaps."
These bivalves possessing a foot present three pairs of ganglia instead of two—the anterior or oval, the posterior or branchial, and the inferior or pedal. It occasionally happens, however, that the ganglia of the posterior or of the inferior pair become approximated or even fused into one. The fusion of the posterior pair takes place, as in the oyster, when the branchiæ from which they receive nerves come close together posteriorly. On the other hand, in those mollusks in which the branchiæ are farther apart, the two ganglia remain separate, and are connected only by a short commissure, as in the mussel (Fig. 9, b).
|Fig. 8.—Nervous System of the Oyster.||Fig. 9.—Nervous System of the Common Mussel.|
The separate existence or fusion of the inferior or pedal ganglia depends upon the size and shape of the foot. The nerves in relation with these ganglia are distributed almost wholly to this organ and its retractor muscles. Where the foot is broad the ganglia remain separate, and are merely connected by a commissure. But where the foot is small and narrow, as in the mussel, the two ganglia become fused into one (Fig. 9, p).
Some of the special senses are unquestionably represented among these headless Mollusca, though the distribution of the different organs is very peculiar. Thus in Pecten, Pinna, Spondulus, the oyster, and many others, very distinct and often pedunculated ocelli are distributed over both margins of the pallium or mantle. These vary in number from forty to two hundred or more, and are in connection with distinct branches of the circumpallial nerves. In the razor-fish, the cockle, Venus, and other bivalves possessing those prolongations of the mantle known as siphon-tubes, the eyes are situated either at the base or on the tips of the numerous small tentacles distributed around the orifices of these tubes, which in those of them living in the sand are often the only parts appearing above the surface. The margins of the mantle are also garnished by a number of short though apparently very sensitive tentacles, in which the creature's most specialized sense of touch seems to reside. Some of these tactile appendages, as well as some of the ocelli, send their nerves to the posterior or parieto-splanchnic ganglia, while those situated on the anterior borders of the mantle communicate with the anterior or oral ganglia. The latter ganglia also receive filaments from the so-called labial appendages, whose function is uncertain, though it has been suggested that they may be organs of taste or smell. Lastly, in close relation with the pedal ganglia or ganglion, there are two minute saccules (Fig. 9, s), to which an auditory function is usually ascribed.
Thus we find among these headless mollusks a distribution of specially impressible parts or sensory organs, such as cannot be paralleled among any other animals. The sense of touch and the sense of sight seem to be more especially in relation with the great posterior ganglia. These sensory functions are, however, to a minor extent shared by the oral ganglia, which are also in relation with parts that may possibly be organs of taste or smell. On the other hand, auditory impressions are invariably brought into relation with the inferior or pedal ganglia. In these headless mollusks, therefore, the functions pertaining to the brain in other animals are distributed in a very remarkable manner, and the anterior ganglia cannot in them be properly regarded as representing such an organ.
The viscera in these lamellibranchs are also in relation with the three pairs of ganglia, and not exclusively with any one of them. Filaments to the intestinal canal and the liver are usually given off from the commissures between the anterior and the posterior ganglia; the genital organs are in connection with filaments coming from the commissures between the anterior and the inferior or pedal ganglia; while the branchiæ are in relation with the ganglia at the posterior part of the body.
There is another interesting class of mollusks—the Pteropoda—which,, in respect of powers of locomotion and the possession of a distinct head, may, if for no other reasons, be said to lead us on from the comparatively sluggish bivalveto the gasteropods and the cephalopods, all of which are distinguished by definite and wide reaching powers of locomotion, and by the possession of a distinct head carrying sense-organs, and a more or less developed brain.
The possession, by many members of this class, of two fin-like muscular expansions attached to the side of the head induced Cuvier to give them the name Pteropoda. Prof. Owen says: "All the species of Pteropoda are of small size; they float in the open sea, often at great distances from any shore, and serve, with the Acalephæ, to people the remote tracts of the ocean. In the latitudes suitable to their well-being, the little Pteropoda swarm in incredible numbers, so as to discolor the surface of the sea for leagues; and the Clio and the Limacina constitute, in the northern seas, the principal article of food of the great whales."
Some of the least highly-organized members of this class, such as the Hyalaceos, are provided with a bivalve shell, and cannot be said to possess a head. They have a simple commencement of the alimentary canal at the anterior extremity of the body; but since this anterior extremity has no tactile appendages and no eyes, and inasmuch as it also contains no cerebral ganglia, it can have no claim to be considered as a head. Their chief nervous centre consists of a flat, somewhat quadrate, sub-œsophageal ganglion, to the anterior angles of which is attached a nervous commissure which extends upward so as to encircle the gullet, though there are no ganglia either on or at the sides of this tube in the usual situation occupied by cerebral ganglia.
In other pteropods devoid of a shell, we meet with a higher organization. Thus in Clio there is a distinct head bearing sensory appendages in the form of two tentacula and two eyes, and containing in its interior a brain. This brain is represented by two connected super-œsophageal ganglia, which are in relation, by means of nerves, with the cephalic sensory organs, and in connection with the sub-œsophageal commissure are the two pedal and two branchial ganglia. The two pairs of ganglia exist separately in Clio and its allies, though they are combined into one quadrate mass in Hyalea. In this latter there are two acoustic vesicles in contact with the anterior part of the great ganglion, while in Clio similar vesicles are in connection with the anterior pair of sub-œsophageal ganglia—that is, with the pair which corresponds with the pedal ganglia of the common bivalve mollusks.
Gasteropods constitute a class of organisms which, in point of numbers, can only be compared with the still more numerously represented class of insects. Their name is derived from the fact that these animals crawl by means of a large muscular expansion stretched out beneath the viscera. The locomotion of the members of this class may be said to be, in the main, dependent upon their own individual efforts, so that, in this respect, they differ widely from the pteropods, whose locomotions are brought about by winds driving them along the surface of the water on which they float.
Some gasteropods are terrestrial, air-breathing animals, though by far the greater number are aquatic, and breathe by means of gills. But being all of them, as Prof. Owen says, "endowed with power to attain, subdue, and devour organic matter, dead and living," we find their nervous system not only better developed, more complex and concentrated, but also in relation with more highly-evolved organs of special sense and exploration. It offers considerable variations in its general arrangement, especially as regards relative positions of ganglia, though these modifications are, to a great extent, referable to differences in the outward configuration of the body.
Some of the differences in external form which are to be met with among gasteropods are well illustrated by the limpet or the chiton, as compared with the snail. Here differences in form coexist with differences in habit, so that we almost necessarily meet with notable variations in the disposition of the principal parts of the nervous system.
In the limpet we find that the two small cerebral ganglia (Fig. 10, a) are widely separated from one another, and lie at the side of the œsophagus. Each receives a rather large nerve from one of the tentacles, and a smaller optic nerve. A commissure connects these cerebral ganglia above the œsophagus with one another, while each of them is also in relation by means of two descending commissures with a series of four connected ganglia forming a transversely-arranged row beneath the œsophagus. Of these the two median ganglia (B) correspond with the pedal, while the two external (C) correspond with the branchial ganglia, though they are here separated from one another by an immensely wide interval.
|Fig. 10.—Nervous System of the Common Limpet.||Fig. 11.—Nervous System of Chilon marmoratus.|
However small and undeveloped the duplex brain of the limpet may be, this organ exists in an even more rudimentary state in some other gasteropods. Thus, in the chiton, which is a close ally of the limpet, and about the most simply organized of all the gasteropods, there are neither tentacles nor eyes, and, as a consequence of this, there are (Fig. 11) no supra-œsophageal ganglia. There is nothing, in fact, to which the term brain can be appropriately applied.
But, if we turn now to the much more active snail, we find the nervous system existing in a more developed and concentrated form. There is (Fig. 12, l) a large ganglionic mass situated over the œsophagus, each half of which receives a considerable bundle of nerve-fibres (f) from the eye (b) of the smaller side, which is situated at the tip of the larger tentacle. It also receives another bundle of nerves (k)
from the small tentacle on each side, which has in all probability a tactile function. The auditory vesicles are here in a new position. They are in immediate relation with the posterior aspect of these ganglia constituting the brain, though in other gasteropods they are, as in bivalve Mollusca, found to be connected with the pedal ganglia. That gasteropods are endowed with a rudimentary sense of smell is now generally admitted by naturalists, though hitherto they have been unable to locate this endowment in any particular organ or surface-region.
The brain of the snail is connected, by means of a triple cord or commissure on each side of the œsophagus, with a still longer double ganglionic mass (m). This latter body, situated beneath the œsophagus, represents the pair of pedal and the pair of branchial ganglia of the bivalve Mollusca. Here nerves are received from the integument and given off to the muscles of the foot, while they are also received and given off from the respiratory and other organs.
In the nautilus and some other representatives of the next class, Cephalopoda, the nervous system attains a development only slightly in advance of that met with among the highest gasteropods, though in the active and predaceous cuttle-fish, and in its near ally, the octopus, we find the nervous system presenting the highest development to be met with among; the sub-kingdom Mollusca.
One of the most striking characteristics of the principal nerve-centres of the cuttle-fish is the fact of the existence of a very large optic ganglion (Fig. 13, 2), in connection with a well-developed eye, on each side. Each optic lobe, according to Lockhart Clarke, is "as large as the rest of the cephalic ganglia on both sides taken together." From each of these lobes an optic peduncle passes inward to join a supra-œsophageal ganglionic mass, which bears on its surface a large bilobed ganglion (1), thought by Clarke to be homologous with the cerebral lobes of fishes. It is connected, by means of two short cords,
with a much smaller bilobed ganglion, known as the pharyngeal (7). This double ganglion receives nerves from what are presumed to be the organs of taste and smell, and gives off nerves to the tongue and powerful parrot-like jaws with which the creature is provided.
The supra-œsophageal mass is connected, by cords at the sides of the œsophagus, with a very large ganglion lying beneath it (4), which is partially divided into an anterior and a posterior division. The anterior division is in relation, by means of large nerves (6), with the feet and tentacles. A commissure also unites it with the pharyngeal ganglion, so that the tentacles and arms are thus able to be brought into correlated action with the jaws. The posterior portion of the sub-œsophageal mass receives nerves from, and also gives off nerves (14) to, the branchiæ and other viscera, as well as to the mantle (13, 13).
The auditory organs and their nerves are also connected with this branchial and pallial ganglion. These organs are lodged in the substance of the cartilaginous framework investing the nerve-ganglia—a structure which seems to answer to a rudimentary skull. The roots of the auditory nerves are probably principally in relation with the pallial portion of the branchio-pallial ganglion. The locomotions of these creatures are largely brought about by contractions of the pallial chamber, though these contractions of the mantle are also subservient to the respiratory function.
The share which the branchio-pallial ganglia take in bringing about and regulating the movements of the cuttle-fish would seem to explain the connection of the auditory nerves with them rather than with the homologues of the pedal ganglion, with which the auditory saccules are in relation in most other mollusks. But, whatever may be the precise explanation of the different connections of the auditory nerves in the cuttle-fish tribe, the fact remains that their connections are still away from the brain proper. They are, as in most other Mollusca and in those insects in which auditory organs are known to occur, in intimate relation with one of the principal motor centres.
This survey of some of the principal forms of the invertebrate brain, brief though it has been, should have sufficed to call attention to the following important facts and inferences:
1. That sedentary animals, though they may possess a nervous system, are often headless, and then have nothing answering to a brain.
2. That where a brain does exist, it is invariably a double organ. Its two halves may be widely separated from one another, though at other times they are fused into a single mass.
3. That the component or elementary parts of the brain in these lower animals are ganglia in connection with some of those special impressible parts or sense-organs, by means of which the animal is brought into harmony with its environment or medium.
4. That the sensory ganglia, which as an aggregate constitute the brain of invertebrate animals, are connected with one another both on the same and on opposite sides of the body, either by continuous growth or by means of commissures.
5. The size of the brain as a whole, or of its several parts, is strictly regulated by the development of the animal's special sense-organs. This is so, because, the more these impressible surfaces become elaborated and attuned to help in discriminating between numerous different external impressions, the larger are the ganglionic masses with which their nerves are in relation.
6. Of the several sense-organs and sensory ganglia whose activity lies at the root of the intellectual and instinctive life (such as it is) of invertebrate animals, some are much more important than others. Two are notable for their greater proportional development, viz., tactile organs and visual organs. The former are soon outstripped in importance by the latter. The visual sense, indeed, and its related nerve-ganglia, attain an altogether exceptional development in the higher insects and mollusks.
7. The sense of taste and that of smell are developed to a much lower extent. It is even difficult to point to distinct organs or impressible surfaces as certainly devoted to the reception of impressions of this kind.
8. The sense of hearing is also developed to a very slight extent. No distinct sense-organ of this kind has been discovered, except in a few insects and in members of the sub-kingdom Mollusca. It is, however, of no small interest to find that, where these organs do exist, the nerves issuing from them are not in direct relation with the brain, but are immediately connected with one of the principal locomotor nerve-centres of the body.
9. The associated ganglia representing the single or double brain are, in animals possessing a head, the centres in which all impressions from sense-organs, save those last mentioned (the auditory), are reflected on to appropriate groups of muscles. This "reflection occurs either at once or after the stimulus has passed through other ganglia, whence it is passed along nerves to those groups or combinations of muscles whose simultaneous or successive contractions give rise to the organism's reply to such impressions. It may be easily understood, therefore, that in all such animals perfection of sense-organs, size of brain, and power of executing varied muscular movements, are intimately related to one another.
10. But a fairly parallel correlation also becomes established between these various developments and that of the internal organs. An increasing visceral complexity is gradually attained. Such increased visceral complexity carries with it the necessity for a further development of nervous communications. The several internal organs have to be brought into more perfect relation with the sensori-motor nervous system, and also with one another, for all joint actions in which two or more of them may be concerned.
11. In invertebrate animals the visceral system of nerves has, when compared with the rest of the nervous system, a greater proportional development than among vertebrate animals. Its importance among the Invertebrata is not dwarfed by the enormous development of the brain and spinal cord, which gradually declares itself among the Vertebrata.
12. Impressions emanating from the viscera and stimulating the organism to movements of various kinds, whether in pursuit of food or of a mate, would, therefore, have a proportionally greater importance as constituting part of the ordinary mental life of invertebrate animals. Movements thus initiated will be found to afford a basis for the development of many so-called instinctive acts.
- In speaking of the nervous system of the lamellibranchs, I have not alluded to certain small accessory ganglia which exist in some of them in relation with peculiar specially developed contractile structures.