CHAPTER XXVI

THE TUNICATES, OR ASCIDIANS

=Structure of Tunicates.=--One of the most singular groups of animals is that known as Ascidians, or Tunicates. It is one of the most clearly marked yet most heterogeneous of all the classes of animals, and in no other are the phenomena of degeneration so clearly shown.

Among them is a great variety of form and habit. Some lie buried in sand; some fasten themselves to rocks; some are imbedded in great colonies in a gelatinous matrix produced from their own bodies, and some float freely in long chains in the open sea. All agree in changing very early in their development from a free-swimming or fish-like condition to one of quiescence, remaining at rest or drifting with the current.

Says Dr. John Sterling Kingsley: "Many of the species start in life with the promise of reaching a point high in the scale, but after a while they turn around and, as one might say, pursue a downward course which results in an adult which displays but few resemblances to the other vertebrates. Indeed, so different do they seem that the fact that they belong here was not suspected until about thirty-five years ago. Before that time, ever since the days of Cuvier, they were almost universally regarded as mollusks, and many facts were adduced to show that they belonged near the acephals (clams, oysters, etc.). In the later years when the facts of development began to be known, this association was looked on with suspicion, and by some they were placed for a short time among the worms. Any one who has watched the phases of their development cannot help believing that they belong here, the lowest of the vertebrate series."

The following account of the structure and development of the Tunicate is taken, with considerable modification and condensation, from Professor Kingsley's chapter on the group in the Riverside Natural History. For the changes suggested I am indebted to the kindness of Professor William Emerson Ritter:

The Tunicates derive their name from the fact that the whole body is invested with a tough envelope or "tunic." This tunic or test may be either gelatinous, cartilaginous, or leathery. In some forms it is perfectly transparent, in others it is translucent, allowing enough light to pass to show the colors of the viscera, while in still others it is opaque and variously colored. The tunic is everywhere only loosely attached to the body proper, except in the region of the two openings now to be mentioned. One of these openings occupies a more or less central position, while the other is usually at one side, or it may even be placed at the opposite end of the body. On placing one of the Ascidians in a glass dish and sprinkling a little carmine or indigo in the water, we can study some of the functions of the animal. As soon as the disturbance is over, the animals will open the two apertures referred to, when it will be seen that each is surrounded with blunt lobes, the number of which varies with the species. As soon as they are opened a stream of water will be seen to rush into the central opening, carrying with it the carmine, and a moment later a reddish cloud will be ejected from the other aperture. From this we learn that the water passes through the body. Why it does so is to be our next inquiry. On cutting the animal open we find that the water, after passing through the first-mentioned opening (which may be called the mouth) enters a spacious chamber, the walls of which are made up of fine meshes, the whole appearing like lattice-work. Taking out a bit of this network and examining it under the microscope, we find that the edges of the meshes are armed with strong cilia, which are in constant motion, forcing the water through the holes. Of course, the supply has to be made good, and hence more water flows in through the mouth. This large cavity is known as the branchial or pharyngeal chamber. It is, according to Professor Ritter, "as we know from the embryology of the animal, the greatly enlarged anterior end of the digestive tract; and as the holes, or stigmata, as they are technically called, are perforations of the wall for the passage of water for purposes of respiration, they are both morphologically and physiologically comparable with the gill openings of fishes." There can be no doubt, therefore, that the pharyngeal sac of Ascidians is homologous with the pharynx of fishes.

Surrounding the mouth, or branchial orifice, just at its entrance into the branchial chamber is a circle of tentacles. These are simple in some genera, but elaborately branched in others.

In close connection with the cerebral ganglion, which is situated between the two siphons, there is a large gland with a short trumpet-shaped duct opening into the branchial sac a little distance behind the mouth. The orifice of the duct is just within a ring consisting of a ciliated groove that extends around the mouth outside the circle of branchial tentacles. On the opposite side of the mouth from the gland the ciliated groove joins another groove which is both ciliated and glandular, and which runs backward along the upper floor of the pharyngeal sac to its posterior extremity. This organ, called the endostyle, is concerned in the transportation of the animal's food through the pharyngeal sac to the opening of the oesophagus. Comparative embryology makes it almost certain that the subneural gland with its duct, described above, is homologous with the hypophesis cerebri of true vertebrates, and that the endostyle is homologous with the thyroid glands of vertebrates.

The water after passing through the branchial network is received into narrow passages and conducted to a larger cavity--the cloacal or atrial chamber. The general relations can he seen from our diagram, illustrating a vertical and horizontal section. From the atrial chamber the water flows out into the external world.

Now we can readily see how in the older works naturalists were misled as to the affinities of the Tunicates. They regarded the tunic as the equivalent of the mantle of the mollusks, while the incurrent and excurrent openings corresponded to the siphons. In one genus, _Rhodosoma_, the resemblance was even stronger, for there the tunic is in two parts, united by a hinge line, and closed by an adductor muscle. How and why these views were totally erroneous will be seen when we come to consider the development of these animals.

At the bottom of the pharnygeal sac is the narrow oesophagus surrounded with cilia, which force a current down into the digestive tract. The branchial meshes serve as a strainer for the water, and the larger particles which it contains fall down until they are within reach of the current going down the oesophagus. After passing through the throat, they come to the stomach, where digestion takes place, and then the ejectamenta are carried out through the intestine and poured into the bottom of the atrial cavity.

The heart lies on the ventral side of the stomach and is surrounded by a well-developed pericardium. The most remarkable fact connected with the circulation is that the heart, after beating a short time, forcing the blood through the vessels, will suddenly stop for a moment and then resume its beats; but, strange to say, after the stoppage the direction of the circulation is reversed, the blood taking an exactly opposite course from that formerly pursued. This most exceptional condition was first seen in the transparent _Salpa_, but it may be witnessed in the young of most genera. We have already referred to the branchial chamber. The walls of this chamber, besides acting as a strainer, are also respiratory organs. The meshes of which they are composed are in reality tubes through which the blood circulates and thus is brought in contact with a constantly renewed supply of fresh water.

The central nervous system in the adults of all except the _Larvacea_ is reduced to a single ganglion placed near the mouth thus indicating the dorsal side. In forms like _Cynthia_ it holds the same relative position with regard to the mouth, but by the doubling of the body (to be explained further on) it is also brought near the atrial aperture, where it is shown in our first diagram.

=Development of Tunicates.=--The sexes are combined in the same individual, though usually the products ripen at different times. As a rule, the earlier stages of the embryo are passed inside the cloacal chamber, though in some the development occurs outside the body. As a type of the development we will consider that of one of the solitary forms, leaving the many curious modifications to be noticed in connection with the species in which they occur. This will be best, since these forms show the relationship to the other vertebrates in the clearest manner.

The egg undergoes a total segmentation and a regular gastrulation. Soon a tail appears, and under the microscope the young embryo, which now begins its free life, appears much like the tadpole of the frog. It has a large oval body and a long tail which lashes about, forcing the animal forward with a wriggling motion. Nor is the resemblance superficial; it pervades every part of the structure, as may be seen from the adjacent diagram. The mouth is nearly terminal and communicates with a gill-chamber provided with gill-clefts. At the posterior end of the gill-chamber begins the alimentary tract, which pursues a convoluted course to the vent. In the tail, but not extending to any distance into the body, is an axial cylinder, the notochord, which here, as in all other vertebrates, arises from the hypoblast; and above it is the spinal cord (epiblastic in origin), which extends forward to the brain, above the gill-chamber. Besides, the animal is provided with organs of sight and hearing, which, however, are of peculiar construction and can hardly be homologized with the corresponding organs in vertebrates. So far the correspondence between the two types is very close, and if we knew nothing about the later stages, one would without doubt predict that the adult tunicate would reach a high point in the scale of vertebrates. These high expectations are never fulfilled; the animal, on the contrary, pursues a retrograde course, resulting in an adult whose relationship to the true vertebrates never would have been suspected had its embryology remained unknown.

After the stage described this retrograde movement begins. From various parts of the body lobes grow out, armed on their extremities with sucking-disks. These soon come in contact with some subaquatic object and adhere to it. Then the notochord breaks down, the spinal cord is absorbed, the tail follows suit, the intestine twists around, and the cloaca is formed, the result being much like the diagram near the head of this section. In forms like _Appendicularia_, little degeneration takes place, so far as is known, the tail, with its notochord and neural chord, persisting through life.

=Reproduction of Tunicates.=--As to the reproduction of the Tunicates, Dr. Ritter writes: "In addition to the sexual method of reproduction, many tunicates reproduce asexually by budding. The capacity for bud reproduction appears to have been acquired by certain simple Ascidians in connection with, probably as a result of, their having given up the free-swimming life and become attached and consequently degenerate.

"Instructive as the embryonic development of the creatures is from the standpoint of evolution, the bud method of development is scarcely less so from the same point of view. The development of the adult zooid from the simple bud has been conclusively shown to be by a process in many respects fundamentally unlike that by which the individual is developed from the egg. We have then in these animals a case in which practically the same results are reached by developmental processes that are, according to prevailing conceptions of animal organizations, fundamentally different. This fact has hardly a parallel in the animal kingdom."

=Habits of Tunicates.=--The Tunicates are all marine, some floating or swimming freely, some attached to rocks or wharves, others buried in the sand. They feed on minute organisms, plants, or animals, occasional rare forms being found in their stomachs. Some of them possess a single median eye or eye-like structure which may not do more than recognize the presence of light. No fossil Tunicates are known, as they possess no hard parts, although certain Ostracoderms have been suspected, though on very uncertain grounds, to be mailed Tunicates, rather than mailed lampreys. It is not likely that this hypothesis has any sound foundation. The group is divided by Herdman and most other recent authorities into three orders, viz., the _Larvacea_, the _Ascidiacea_, and the _Thaliacea_.

=Larvacea.=--In the most primitive order the animals are minute and free-swimming, never passing beyond the tadpole stage. The notochord and the nervous chord persist through life, the latter with ganglionic segmentations at regular intervals. The species mostly float in the open sea, and some of them form from their own secretions a transparent gelatinous envelope called a "house." This has two apertures and a long chamber "in which the tail has room to vibrate."

The order consists of a single small family, _Appendiculariidæ_. The lowest type is known as _Kowalevskia_, a minute creature without heart or intestine found floating in the Mediterranean. It is in many respects the simplest in structure among _Chordate_ animals. _Oikopleura_ (Fig. 288) is another genus of this group.

=Ascidiacea.=--In the _Ascidiacea_ the adult is usually attached to some object, and the two apertures are placed near each other by the obliteration of the caudal area. The form has been compared to a "leathern bottle with two spouts."

The suborder _Ascidiæ simplices_ includes the solitary Ascidians or "sea-squirts," common on our shores, as well as the social forms in which an individual is surrounded by its buds. The common name arises from the fact that when touched they contract, squirting water from both apertures. The _Ascidiidæ_ comprise the most familiar solitary forms, some of them the largest of the Tunicates and represented on most coasts. In the _Molgulidæ_ and most _Ascidiæ compositæ_ the young hatch out in the cloaca, from which "these tadpoles swim out as yellow atoms," while in a new genus, _Euherdmania_, described by Ritter, from the coast of California, the embryos are retained through their whole larval stage in the oviduct of the parent. They form, according to Kingsley, adhesive processes on the body, but those of _Molgula_ cannot use them in becoming attached to rocks, since they are entirely inclosed in a peculiar envelope. This envelope is after a while very adhesive, and if the little tadpole happens to touch any part of himself to a stone or shell he is fastened for life. Thus "I have frequently seen them adhere by the tail, while the anterior part was making the most violent struggles to escape. Soon, however, they settle down contentedly, absorb the tail, and in a few weeks assume the adult structure."

In the family _Cynthiidæ_ the brightly-colored red and yellow species of _Cynthia_ are known as sea-peaches by the fishermen. The sea-pears, _Boltenia_, are fastened to long stalks. These have a leathery and wrinkled tunic, to which algæ and hydroids freely attach themselves. Into the gill-cavity of these forms small fishes, blennies, gobies, and pearl-fishes often retreat for protection.

The social Ascidians constitute the _Clavellinidæ_. They are similar to the _Ascidiidæ_ in form, but each individual sends out a bud which forms a stern bearing another individual at the end. By this means large colonies may be formed.

The suborder, _Ascidiæ compositæ_, contains the compound Ascidians or colonies enveloped in a common gelatinous "test." These colonies are usually attached to rock or seaweed, and the individuals are frequently regularly and symmetrically arranged. The bodies are sometimes complex in form.

In the _Botryllidæ_ and _Polystyelidæ_ the individuals are not segmented and in the former family are arranged in star-shaped groups about a common cloaca, into which the atrial siphons of the different individuals open. The group springs by budding from the tadpole, or larva, which has attached itself to some object. These forms are often brightly colored. _Botryllus gouldi_ is a species very common along our North Atlantic coast, forming gray star-shaped masses sometimes an inch across on eel-grass (_Zostera_) and on flat-leaved seaweeds. _Goodsiria dura_, a representative of the _Polystyelidæ_, is one of the most common Ascidians on the California coast southward, where the brick-red masses incrusting on seaweeds of various kinds, and on other Ascidians, are frequently thrown ashore in great quantities during heavy storms.

In _Didemnidæ_ the body is more complex, of two parts, called the "thorax" and "abdomen." In _Amaroecium_, the "sea pork" of the fishermen, the body is in three parts and the individuals are very long. These sometimes form great masses a foot or more long, "colored like boiled salt pork, but more translucent." Other families of this type are the _Distomidæ_ and the _Polyclinidæ_.

In the suborder _Luciæ_, including the family _Pyrosomidæ_, the colonies are thimble-shaped and hollow, the incurrent openings being on the outer surface of the thimble, the outgoing stream opening within. _Pyrosoma_ is highly phosphorescent. In the tropical seas some colonies reach a length of two or three feet. It is said that a description of a colony was once written by a naturalist on a page illumined by the colony's own light. "Each of the individuals has a number of cells near the mouth the function of which is to produce the light."

=Thaliacea.=--In the order _Thaliacea_ the Tunicates have the two orifices at opposite ends of the body. All are free-swimming and perfectly transparent. The principal family is that of _Salpidæ_. The gill-cavity in Salpa is much altered, the gills projecting into it dividing it into two chambers.

In these forms we have the phenomena of alternation of generations. A sexual female produces eggs, and from each hatches a tadpole larva which is without sex. This gives rise to buds, some at least of the individuals arising which in turn produce eggs.

In the family _Salpidæ_ two kinds of individuals occur, the solitary salpa, or female, and the chain salpa, or bisexual males. The latter are united together in long bands, each individual forming a link in the chain held together by spurs extending from one to the next. From each solitary individual a long process or cord grows out, this dividing to form the chain. Each chain salpa produces male reproductive organs and each develops as well a single egg. The egg is developed within the body attached by a sort of placenta, while the spermatozoa are cast into the sea to fertilize other eggs. From each egg develops the solitary salpa and from her buds the chain of bisexual creatures. Dr. W. K. Brooks regards these as nursing males, the real source of the egg being perhaps the solitary female. Of this extraordinary arrangement the naturalist-poet Chamisso, who first described it, said: "A salpa mother is not like its daughter or its own mother, but resembles its sister, its granddaughter, and its grandmother." But it is misleading to apply such terms taken from the individualized human relationship to the singular communal system developed by these ultra-degenerate and strangely specialized Chordates.

The Salpas abound in the warm seas, the chains often covering the water for miles. They are perfectly transparent, and the chains are often more than a foot in length. In Doliolum the body is barrel-shaped and the gills are less modified than in Salpa. The alternation of generations in this genus is still more complicated than in Salpa, for here we have not only a sexual and a non-sexual generation, the individuals of which differ from each other, but there is further a differentiation among the asexually produced individuals themselves; so that we have in all three instead of two sorts of animals in the complete life cycle. Besides the proliferating stolon situated on the ventral side, the bud-producing individual possesses a dorsal process larger than the stolon proper. The buds become completely severed from the true stolon at an early stage and actually crawl along the side of the parent up to the dorsal process, upon which they arrange themselves in three rows, two lateral and one median. The buds of the lateral rows become nutritive and respiratory zooids, while those of the median row, ultimately at least, give rise in turn to the egg-producing individuals.

=Origin of Tunicates.=--There can be little doubt that the _Tunicata_ form an offshoot from the primitive Chordate stock, and the structure of their larva in connection with that of the lancelet throws a large light on the nature of their common parents. "We may conclude," says Dr. Arthur Willey, "that the proximate ancestor of the Vertebrates was a free-swimming animal intermediate in organization between an Ascidian tadpole and Amphioxus, possessing the dorsal mouth, hypophysis, and restricted notochord of the former and the myotomes, coelomic epithelium, and straight alimentary canal of the latter. The ultimate or primordial ancestor of the Vertebrates would, on the contrary, be a worm-like animal whose organization was approximately on a level with that of the bilateral ancestors of the Echinoderms."

=Degeneration of Tunicates.=--There is no question, furthermore, Professor Ritter observes, "that most of the group has undergone great degeneration in its evolutionary course. Just what the starting-point was, however, is a matter on which there is considerable difference of opinion among authorities. According to one view, particularly championed by Professor W. K. Brooks, _Appendicularia_ is very near the ancestral form. The ancestor was consequently a small, marine, free-swimming creature. From this ancestor the Ascidiacea were evolved largely through the influence of the attached habit of life, and the tadpole stage in their development is a recapitulation of the ancestral form, just as the tadpole stage in the frog's life is a repetition of the fish ancestry of the frog.

"According to the most common view _Appendicularia_ is not an ancestral form at all, but is the tadpole stage of the _Ascidiacea_ that has failed to undergo metamorphosis and has become sexually mature in the larval condition, as the larva of certain Amphibians and insects are known to never pass into the adult state but reproduce their kind sexually in the larval condition. By this view the tadpole of such Ascidian as _Ciona_, for example, represents more closely the common ancestor of the group than does any other form we know. This view is especially defended by Professor K. Heider and Dr. Arthur Willey."