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.
[Illustration: FIG. 278.--Development of the larval Tunicate to the
fixed condition. (From Seeliger, per Parker & Haswell.) _a_, larva;
_b_, intermediate stage; _c_, adult.]
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.
[Illustration: FIG. 279.--Anatomy of Tunicate. (After Herdman, per
Parker & Haswell.)]
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."
[Illustration: FIG. 280.--_Ascidia adhærens_ Ritter. Glacier Bay,
Alaska. (After Ritter.)]
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.
[Illustration: FIG. 281.--_Styela yacutatensis_ (Ritter), a simple
Ascidian. Family _Molgulidæ_. Yakutat Bay, Alaska. (After Ritter.)]
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.
[Illustration: FIG. 282.--_Styela greeleyi_ Ritter. Family _Molgulidæ_.
Lukanin, Pribilof Islands. (After Ritter.)]
[Illustration: FIG. 283.--_Cynthia superba_ Ritter. A Tunicate from
Puget Sound. Family _Cynthiidæ_. (After Ritter.)]
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.
[Illustration: FIG. 284.--_Botryllus magnus_ Ritter. A compound
Ascidian. Shumagin Islands, Alaska. (After Ritter.)]
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.
[Illustration: FIG. 285.--_Botryllus magnus_ Ritter. Part of colony.
(After Ritter.)]
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.
[Illustration: FIG. 286.--_Botryllus magnus_ Ritter, a single Zooid.
Shumagin Islands, Alaska. (After Ritter.)]
[Illustration: FIG. 287.--_Aplidiopsis jordani_ Ritter, a compound
Ascidian. Lukanin Beach, Pribilof Islands. (After Ritter.)]
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."
[Illustration: FIG. 288.--Adult Tunicate of the group Larvacea,
Oikopleura. Family _Appendiculariidæ_. (After Fol, per Parker &
Haswell.)]
=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."