The class Cyclostomata consists of two orders, the Myxinoids
(or Hyperotreti) and the Petromyzontes (or Hyperoartii),
which, while showing sufficient resemblance in structure to
warrant their inclusion in the same class, are yet marked off by
such deep-seated differences as to indicate that they commenced
to diverge from one another far back in evolutionary time. The
order Myxinoids includes the hagfish (Myxine), common off the
eastern, and occurring also, though less commonly, off the western
coasts of the north Atlantic, and the genus Bdellostoma (also
known as Homea, Eptatretus, in part—Polistotrema), including
the “borers” of the western American coast, New Zealand and
the Cape of Good Hope. The order Petromyzontes includes
the widely distributed lampreys. The original genus Petromyzon
(which it is now customary to subdivide into a number of genera)
includes the large sea lamprey (P. marinus) of the north Atlantic
coasts and the two fresh-water lampreys of European streams
(P. fluviatilis and P. planeri, the latter of which is possibly only
a small-sized variety of the former species). In North America
nine or ten species of lampreys are known to occur, descriptions of
which are given by Jordan and Evermann (1). In the southern
hemisphere occur the two genera Mordacia (Chile, Tasmania)
and Geotria (Chile, Australia, New Zealand) (2).
The Cyclostomes are remarkable among vertebrates in that they are semiparasitic in habit. The lampreys—except some of the small fresh-water forms—attach themselves to other fishes by their suctorial mouth and proceed to rasp off the flesh by means of the horny teeth carried by the highly-developed tongue. The Myxinoids have gone a step further and actually bore their way right into the body of their prey, devouring all the soft parts and leaving the skin behind as a mere shell, empty but for the bones. Where the hagfish or borers are abundant, as in certain localities off the east coast of Scotland and off the west coast of California, they may do great damage to fisheries from their habit of attacking fishes which are in difficulties through being caught by a hook or in a net; the fish when drawn up being frequently completely deprived of their flesh.
The Myxinoids retain the ancestral marine habitat, but the lampreys have sought refuge from the struggle for existence by taking to fresh water to a less or greater extent. Such a form as Petromyzon marinus or Entosphenus tridentatus of the west coast of America is what is known as anadromous in habit, i.e. it takes refuge in fresh water during the breeding season, ascending rivers like the salmon for the purpose of spawning. Certain species of lampreys, on the other hand, have completely deserted the sea and spend their whole lives in fresh-water streams or lakes. The lake lampreys show a reminiscence of their ancestral migratory habits in leaving lakes and ascending streams in order to deposit their spawn.
Anatomy.—In structural features, the Cyclostomes show a curious mixture of features which must be looked on as primitive with others which are indicative of high specialization for their peculiar mode of life. In general appearance they are “eel-like”: they are elongated in shape and adapted for swimming in eel fashion, i.e. the body is propelled forward by the backward passage along it of waves of lateral flexure. There are, however, certain conspicuous differences which at once serve to distinguish a Cyclostome from any other fishes of eel-like shape:—(1) the circular permanently open mouth, (2) the absence of all trace of paired limbs, (3) the absence of paired external nasal openings, and (4) the presence on the roof or at the tip of the head of a conspicuous median opening—the pituitary opening.
It will be convenient, in describing the structural features of the group, to take as a basis for the description the marine lamprey, Petromyzon marinus. A marine lamprey is an eel-like creature 70 to 75 cm. in length. At the anterior end and situated somewhat ventrally is the circular widely gaping mouth or buccal cavity, its lining studded with sharply pointed thorn-like “teeth” and its edge fringed with numerous sensory papillae. On the dorsal side of the head is the conspicuous circular pituitary opening with prominent lips, while on the sides are seen the eyes, and behind these a row of somewhat rounded branchial openings or gill-clefts. At about the beginning of the posterior fourth of the body, and in the midventral line, is the anal opening, and immediately behind it is the prominent papilla carrying the opening of the urogenital sinus. The hinder portion of the body, in accordance with its function in locomotion, is flattened from side to side, while its surface is increased by the development of a median fin fold, divided, except in early stages of development, into three portions, known as the first and second dorsal fins and the caudal fin. The last mentioned is of the primitive protocercal type. The whole surface of the body—which shows a conspicuous dark marbling, especially dorsally, on a light ground—is covered with highly glandular epidermis. An important feature is the complete absence of all trace of the calcified placoid plates which are so characteristic of the Elasmobranchii.
The Myxinoids differ from the lampreys in regard to several of the above-mentioned characters. The edges of the mouth carry tentacle-like barbels. The pituitary opening is close to the anterior edge of the mouth opening instead of being right up on the dorsal side of the head. The eyes are invisible, being greatly reduced and sunk far below the surface, and in Myxine, though not in Bdellostoma, the row of gill openings is represented by a single opening on each side nearly in the midventral line and situated at about the end of the first quarter of the body length. Ventrally the Myxinoid possesses on each side of the body a row of remarkable epidermal glands which can produce at will enormous quantities of glutinous slime. This secretion, which, no doubt, is of much value as a protection from attack, is composed of very fine threads, formed by the conversion of the protoplasm of certain cells of the epidermal glands (“thread cells”) into an extremely fine, tightly coiled filament, which becomes unwound when discharged to the exterior.
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| From D. Starr Jordan, A Guide to the Study of Fishes, by permission of A. Constable & Co., Ltd. |
| Fig. 1.—The Marine Lamprey (Petromyzon marinus, L.). |
Pituitary Tube.—A remarkable peculiarity of the Cyclostomes lies in the fact that the pituitary ingrowth of ectoderm does not, as in other forms, become involved in the inpushing of ectoderm which forms the buccal cavity. On the contrary, it lies outside the edge of the stomodaeum, and in the case of the lampreys active growth takes place in the tissue between the pituitary and stomodaeal ingrowths, so that the two openings come to be widely separated, the pituitary opening being pushed back on to the dorsal side of the head. The pituitary opening remains patent throughout life, as is the case with Crossopterygians alone amongst Gnathostomata. In Myxine a further remarkable peculiarity in regard to the hypophysis, probably adaptive in nature, occurs, inasmuch as the pituitary invagination develops an opening at its posterior end into the pharynx.
Nervous System.—The anterior end of the nervous tube is enlarged and differentiated to form a brain as in other Vertebrates, but this brain in the lampreys at least shows remarkably primitive features. The enlargement as compared with the spinal cord is seen to be comparatively slight: the brain is much elongated, and its various regions lie in a straight line one behind the other: the roof of the brain retains to a great extent the primitive epithelial condition. On each side anteriorly there is present a comparatively large olfactory lobe, and this is continued posteriorly into a small cerebral hemisphere.
The lampreys are amongst those vertebrates in which there is an eye-like apparatus (3) connected with the roof of the thalamencephalon. There grow out from the roof of the thalamencephalon two processes, a posterior (the pineal process), and an anterior (the parapineal process). The pineal process grows forwards so as to overlie the parapineal process. Each of these projections from the roof of the thalamencephalon dilates to form a vesicle, and each vesicle shows certain eye-like characteristics, its deep wall forming a “retina” and its superficial wall being clear and translucent (“pellucida”). The retinal cells are packed in the case of the pineal organ with opaque white pigment: similar pigment occurs in smaller quantity in the parapineal organ. Definite sensory cells are also present with rod-like structures projecting into the lumen of the vesicle. Nerve fibres have been traced—from the pineal organ into the posterior commissure and possibly into the right habenular ganglion. As regards other parts of the brain, the chief point to note is that the cerebellum is in a most rudimentary condition, forming merely a slight transverse thickening of the hind-brain roof at its anterior end. In Myxinoids the brain is much larger as compared with the spinal cord, and it differs from that of the lampreys by being relatively much shorter in an anteroposterior direction. A remarkable negative feature lies in the complete absence of the pineal and parapineal organs so conspicuous in the lampreys. The olfactory organ of Cyclostomes is remarkable for two special characteristics, firstly, that the two olfactory organs of other vertebrates are here represented by a single median structure, and secondly,
