Natural History, Mollusca/Tunicata

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Natural History, Mollusca by Philip Henry Gosse
Tunicata

CLASS VI. TUNICATA.

(Ascidian Mollusks.)


The calcareous shell entirely disappears in the animals of this Class, their organs being enclosed in a case of leathery substance, more or less thick and tough, usually called the tunic or test. In general, they bear a close resemblance to the Conchifera, if the latter be deprived of their protecting valves, as has already been noticed in speaking of the Myadæ. The ordinary form which they assume is that of a leathery, usually semi-pellucid sac, with two openings, the one for the entrance, the other for the exit, of the nourishing water. The gills never appear in the form of free leaves, but constitute a kind of bag, the surface of which is covered with oblong cells or depressions, lined with cilia. Their circulatory system has this remarkable peculiarity, that it ebbs and flows: the blood, being driven from the heart through the vessels in one direction for a certain time, suddenly stops, and after a few moments pursues a retrograde course for a like period.

All the Tunicate Mollusca are marine; many of them are permanently fixed to other substances, others are permanently free; all, however, have the power of locomotion during the infancy of their existence. Some of the genera are single and isolated; others are social, always living aggregated in groups; and others are united into masses, organically possessing a compound life. Through these last-named the transition is easy and natural to the lowest forms of animal life, the Polypes; for the Class of animals denominated Bryozoa or Polyzoa, which form the exquisite aggregations of calcareous cells known as Sea-mats (Flustra), and their allies, are truly Ascidians in their structure, with their entering orifice surrounded by a radiated circle of ciliated tentacles.


Family Ascidiadæ.

Whoever has turned over stones at the extreme verge of low-water (and there is no student of marine natural history who has not) is familiar with uncouth pellucid bodies adhering to their under surfaces, that resemble bags, of a substance somewhat between leather and jelly. The dredger, however, is far more familiar with them, for scarcely can any part of the bottom of the sea be raked without an abundance of these curious creatures being brought up, varying much in size and colour. Some are sufficiently rude and uncouth, coarse and rough in texture, and dingy in hue; others are attractive, often of brilliant colours, of a semi-transparent clearness, resembling strange pellucid fruits, or masses of ice. They are always found adherent, either by the base of the sac or by its side, to foreign bodies as stones, shells, and sea-weeds, and thus not only are they deprived of locomotion, but almost of all appearance of vitality, for no movement is perceptible in them externally, except the periodical opening and closing of the two orifices which give admission and exit to the currents of water for respiration and food.

The branchial sac occupies the upper part of the animal; its interior surface is divided into cells by plaits or ridges, which are clothed with cilia; its orifice is surrounded by a circle of tentacles, and its inferior extremity merges into the digestive tube. For as the food of these animals consists of microscopic organisms, which are drawn in by the entering currents, the same influx of water brings oxygen for the respiration, and food for the nutriment of the system.

Natural History - Mollusca - Ascidia virginea.png

ASCIDIA VIRGINEA.

The water, in yielding its vital properties, passes towards the bottom of the body, and then returning by an upward course is discharged through an orifice closely resembling the other in appearance, and situated near to it on one side. Both orifices can be completely closed at will, by being drawn together in wrinkles, and each is commonly surrounded with minute coloured specks, that are considered to represent eyes. These specks are usually eight around the receiving orifice, and six around the discharging one. The species of this family are very numerous: nearly fifty are enumerated as British; and probably not half of the whole number have yet been described. One of the most common is Ascidia virginea, which grows to an inch and a half in length, and about three quarters in breadth. It is pellucid and crystalline, of a pale yellowish tinge, revealing through the test the branchial sac, spotted with crimson, and crossed with lines of white. It adheres to stones, dead shells, and living seaweeds in deep water.


Family Clavelinadæ.

(Social Ascidians.)

In essential points these resemble the preceding family, but the individuals are not distinct, but united by a common root-thread, from which they spring, like buds from a creeping root-stock. The thread creeps over the surfaces of stones, the stems of sea-plants, &c., continually growing by a lengthening of its extremity, and increasing by throwing out, either in groups or at regular intervals, a kind of buds, that develop into Ascidian mollusks, which commonly stand on more or less distinct foot-stalks.

The family may be illustrated by the accompanying figure, greatly magnified, of a tiny species (Perophora Listeri), found occasionally on our own coasts. I obtained the individual from which the figure was taken at Ilfracombe, attached to a fragment of sea-weed. It is, to the naked eye, a globule of clear jelly, not larger than a pin's head, yet disclosing, under the microscope, an elaborate system of organs for respiration, circulation, and digestion.

Natural History - Mollusca - Perophora.png

PEROPHORA.

The rows of oval rings are the cells of the branchial sac, and the arrows show the course of the blood as it circulates from the oblong heart at the bottom of the body all around the system.

Family Botryllidæ.

(Starry Ascidians.)

The stones that are ordinarily covered by the tide, and the coarser sea-weeds, such as the Fuci and Laminariœ, that grow at low-water, are very frequently studded with irregular patches of dark-coloured substance, gelatinous to the feel, and often somewhat brightly coloured, the more common hues being blue, purple, green, grey, and white. On closer examination, we find embedded in this mass, circles of stars, each consisting of a definite number of bright-hued, minute, oblong bodies, radiating from a common centre.

These masses belong to the genus Botryllus, the representative of the family before us; and each radiating point is an individual animal. From ten to twenty of such animals are ordinarily grouped together to form one of the wheel-like systems; and there are often as many systems in one encrusting mass.

The organization of these little animals is in general conformable to what has been already described; but the discharging orifice of each individual is placed at the opposite end from the mouth, and opens into the common centre, which, rising with a circular rim, expands and contracts as a discharging orifice for the whole of that system.

Several species of this genus are common with us, one of the most abundant of which is the Botryllus polycyclus, which is found encrusting the broad leaves of the common Tangle (Laminaria digitata). It is of a bluish purple hue, with the individuals marked by white rays.

Other genera in this family form similar encrusting masses, but the animals are placed in irregular tortuous lines instead of stars, and the two orifices are near together. Others do not encrust foreign substances, but are grouped in variously shaped knots, or fruit-like bodies, adhering to stones and shells.


Family Salpadæ.

(Swimming Ascidians.)

The body in this group is free, or not adherent; more or less cylindrical; with a thick external envelope, which is somewhat cartilaginous; transparent; having the two orifices, which are ordinarily very large and distant, nearly terminal, one at each extremity. The branchiæ, in the form of a narrow band, traverse obliquely the respiratory cavity of the receiving orifice to the aperture of the mouth.

M. de Blainville remarks, that one may easily perceive the relationship of this family to the other Tunicata, by supposing an Ascidia slit between the two tubes which terminate it, and then extended lengthwise. It is then, he observes, easy to determine the analogy of the apertures, of which neither the one nor the other is properly any more the mouth or the anus than in the Ascidiæ; but one (the widest, the greatest, and the most distant from the mouth), is the entrance of the incretory or respiratory tube, and the other is that of the excretory tube. He adds, that the species of this family are, like those of the preceding, susceptible of living solitary, or aggregated in a fixed manner, which would seem to make them composite animals, though they are not such; and he divides it into two tribes:—1. The simple Salpaceans (Salpa); 2. The aggregate Salpaceans (Pyrosoma).

"A great interest," observe Messrs. Forbes and Hanley, "is attached to the natural history of the Salpæ, on account of their singular mode of reproduction, discovered by the German naturalist Chamisso, and the extraordinary generalization to which that discovery in a great measure gave rise. Previous observers had noticed that these animals were sometimes found solitary, at others united together in long chains, composed of numerous individuals of similar form, each an independent being, though constantly associated, and linearly aggregated with its companions. These long chains swim through the tranquil water with regular serpentine movements; for the creatures of which they are composed contract and expand simultaneously, keeping time, as it were, like a regiment of soldiers upon parade. Each chain seems, consequently, to be a single being, acting through the influence of an unique will; and hence sailors often look upon it as a reptile; and in many seas the Salpa-chains are called sea-serpents. But when taken out of the water, the links of the chain fall asunder, the several distinct animals of which it is composed suddenly losing their power of adhesion. In consequence of accidents, broken-up chains and separated members of such communities are not unfrequently met with, in seas where Salpæ are numerous. But other Salpæ are also met with, very dissimilar in form, and never united together in chains. Now, the discovery of Chamisso was, that such constantly solitary Salpæ did not belong to species distinct from those united in chains, however dissimilar (and they are so dissimilar usually, as to appear even generically distinct), but were either the parents or the progeny, as the case might be, of the aggregate forms; that chained Salpæ did not produce chained Salpæ but solitary Salpæ which, in their turn, did not produce solitary beings, but chained. Consequently, as Chamisso graphically observes, 'a Salpa mother is not like its daughter or its own mother, but resembles its sister, its grandaughter, and its grandmother.'"[1]

More recent researches have fully confirmed the correctness of these observations, strange as they at first appeared. Nor are the facts so singular as they were then believed to be; for the same law (now known as that of the Alternation of Generations), has been found to prevail extensively in the Medusæ and Hydroid Polypes.

One or more species of this genus have been at various times observed in the seas which wash the British coasts; and the first detection of the genus we owe to the eminent geologist, Dr. McCulloch. His graphic description of the discovery is so interesting, that I shall give it with a slight abridgement, though it repeats some details already mentioned. The species was probably Salpa runcinata.

"Some marine animals occur in these seas which remain still unrecorded in the catalogue of British zoology. Among these, indeed, it is probable that a few will be found still undescribed by naturalists, since fresh additions are even yet occasionally made to our catalogue of these obscurer parts of the creation. Many of these animals have occasionally fallen under my notice; but amid pursuits which rendered it impossible to attend either to their examination or preservation. I have, however, preserved a memorial of one, as it appears to form a new species, in a tribe of which no individual has yet been observed within the limits of the British seas. It belongs, apparently, to the genus Salpa. . . .

"The mode in which the republic is linked together, is observed to be constant in each species; and it is sufficiently remarkable in this one, to distinguish it from the rest of the genus, as far as it is yet described. Each individual adheres to the preceding, by a regular sequence of superposition, lengthwise; so that the whole forms a long, simple chain, the adhesion continuing as in the ovarium, for some time after hatching. They were found from the middle to the latter end of August, and always linked together. It is probable that their separation takes place at a later season of the year, but I did not observe them in that state. The individual is amongst the most simple in shape of those yet described, presenting an oval-lanceolate and slightly rhomboidal flattened figure, without appendages. The anal opening is of a bright brown hue, and circular, being placed at some distance from the extremity; and when the chain is linked together, all these apertures are directed the same way. The animal is perfectly hyaline, and tender, and the adhesion of the chain so slight, that the individuals are easily separated. The act of swimming is known to result from the introduction and emission of water by each animal: and as the republic swims together by an undulating motion resembling that of a serpent—the chain often extending to many feet in length—it is evident that this motion must arise from the unequal manner in which the different individuals act throughout the whole line. . . .

"I had occasion to remark of this animal, that, like the Medusæ and analogous tribes, it cannot bear to be confined in a limited portion of water, as it died, even in the ship's bucket, in less than half an hour—a very remarkable circumstance in the economy of these imperfect animals.

"Hitherto, this genus is only known as the inhabitant of hot climates, and of the Mediterranean Sea. I found it in great abundance in the harbours of Canna and Campbelltown, rising to the surface in calm weather, and crowding the water as the Medusæ often do at the same time of the year. . . .

"I was desirous of observing whether this animal, like many other of the marine worms, emitted light, but had no opportunity of ascertaining the fact, as they seemed always to retire to the bottom at sunset, and those which were taken on board died (as I have already observed), in a very short time."[2]

The Pyrosomata are aggregated in another manner. They consist of lengthened individual Ascidians, united to each other at their basal part, and free at the opposite extremity, with their connexion so arranged as to compose numerous and regular rings, which concur to form a long, free cylinder, or tube, open at one extremity and closed at the other.

Cuvier states, that this great cylinder swims in the sea by means of the combined contractions and dilatations of all the individual animals which compose it. The branchial orifices are pierced near the points, and the anus opens into the interior cavity of the tube. Thus, says Cuvier, one may compare a Pyrosoma to a great number of stars of Botrylli, strung one after the other, but the whole of which would be moveable.

Mr. George Bennett, in his interesting "Wanderings in New South Wales," after some valuable remarks on the luminosity of the ocean, proceeds as follows:—

"On the 8th of June, being then in latitude 30' south, and longitude 27° 5' west, having fine weather and a fresh south-easterly trade-wind, and the range of the thermometer being from 78° to 84°, late at night, the mate of the watch came and called me to witness a very unusual appearance in the water, which he, on first seeing it, considered to be breakers. On arriving upon the deck, this was found to be a very broad and extensive sheet of phosphorescence, extending in a direction from east to west, as far as the eye could reach. The luminosity was confined to the range of animals in this shoal, for there was no similar light in any other direction. I immediately cast the towing-net over the stern of the ship, as we approached nearer the luminous streak, to ascertain the cause of this extraordinary and so limited phenomenon. The ship soon cleaved through the brilliant mass, from which, by the disturbance, strong flashes of light were emitted; and the shoal, judging from the time the vessel took in passing through the mass, may have been a mile in breadth. The passage of the vessel through them increased the light around to a far stronger degree, illuminating the ship. On taking in the towing-net, it was found half filled with Pyrosoma (Atlanticum?), which shone with a beautiful pale-greenish light; and there were also a few shell-fish in the net at the same time. After the mass had been passed through, the light was still seen astern, until it became invisible in the distance; and the whole of the ocean then became hidden in darkness as before this took place. The scene was as novel as beautiful and interesting; more so from my having ascertained, by capturing luminous animals, the cause of the phenomenon.

"The second occasion of my meeting these creatures was not exactly similar to the preceding; but though also limited, was curious, as occurring in a high latitude, during the winter season. It was on the 19th of August, the weather dark and gloomy, with light breezes from north-north-east, in latitude 40° 30' south, and longitude 138° 3' east, being then distant about 368 miles from King's Island (at the western entrance of Bass's Straits). It was about eight o'clock p.m. when the ship's wake was perceived to be luminous; and scintillations of the same light were also abundant around. As this was unusual, and had not been seen before, and it occasionally, also, appeared in larger and smaller detached masses, giving out a high degree of brilliancy,—to ascertain the cause, so unusual in high latitudes during the winter season, I threw the towing-net overboard, and in twenty minutes succeeded in capturing several Pyrosomata, giving out their usual pale-green light; and it was, no doubt, detached groups of these animals that were the occasion of the light in question. The beautiful light given out by these molluscous animals soon ceased to be seen emitted from every part of their bodies; but by moving them about, it could be reproduced for some length of time after. As long as the luminosity of the ocean was visible (which continued most part of the night), a number of Pyrosoma Atlanticum, two species of Phyllosoma, an animal apparently allied to Leptocephalus, as well as several crustaceous animals (all of which I had before considered as intertropical species), were caught and preserved. At half-past ten p.m. the temperature of the atmosphere on deck was 52°, and that of the water 51½°. The luminosity of the water gradually decreased during the night, and towards morning was no longer seen, nor on any subsequent night."[3]


  1. Brit. Moll. i. 47.
  2. Western Isles, ii. 187.
  3. Wanderings, vol. i.