The anterior end dilates to form the thin-walled cerebral vesicle (see
Figs. 25, I, and 26, A), containing later the intra-cerebral, dorsal,
pigmented eye (_oc_), and the ventral otolith (_au_) of the larva. The next
part of the canal thickens to form the trunk-ganglion, and behind that is
the more slender "spinal cord," which runs to the extremity of the tail. A
ciliated diverticulum of the anterior end of the enteric cavity (future
pharynx) which enters into close relations with the front of the cerebral
vesicle,[97] and later opens into the ectodermic invagination which forms
the mouth at that spot, is evidently the rudiment of the neural duct or
hypophysial canal. The future branchial sac (pharynx), with a ventral
median thickening which will be the endostyle, is by this time clearly
distinguishable by its large size {59}from the much narrower posterior part
of the enteron, which grows out to become the oesophagus, stomach, and
intestine. The notochord does not extend forward into the pharyngeal
region, but is confined to the posterior or caudal part of the embryo. It
now shows lenticular pieces of a gelatinous intercellular substance
secreted by the cells and lying between them (Fig. 25, I). The mouth forms
as a stomodaeum, or ectodermal invagination, antero-dorsally in the region
where the neuropore has closed, and about the same time two lateral
ectodermal involutions form (Fig. 26, A, _at_), which become the atrial or
peribranchial pouches, at first distinct, afterwards united in the
mid-dorsal line to form the adult cloaca and atrial aperture. Ingrowths
from the atrial pouches and outgrowths from the wall of the pharynx
coalesce to form the proto-stigmata (primary gill-slits) by which the
cavity of the branchial sac is first placed in communication with the
exterior through the atrial apertures. Opinions differ as to whether only
one or a few pairs of true gill-clefts are represented in the young
Ascidian; and the actual details of their formation and subdivision, to
form the stigmata of the adult, differ considerably in different forms. In
_Clavelina_ the stigmata are formed as independent perforations of the
pharyngeal wall; in _Ascidia_ two pairs of protostigmata increase to six
pairs, which are subdivided into stigmata; _Botryllus_ and other forms are
intermediate in some respects. No doubt the subdivision of proto-stigmata
is primitive, but has been lost from the ontogeny in some cases. To what
precise extent the walls of the atrial or peribranchial cavities are formed
of ectoderm, or of endoderm, is still doubtful.
The embryo is hatched about two or three days after fertilisation, as a
larva or Ascidian tadpole (Fig. 26, A) which leads a free-swimming
existence for a short time, during which it develops its nervous system and
cerebral sense-organs, and the powerful mesoblastic muscle-bands lying at
the sides of the notochord (now a cylindrical rod of gelatinous nature
surrounded by the remains of the original cells) in the tail which form the
locomotory apparatus. Fig. 26, A, shows this stage, the highest in its
chordate organisation, when the larva swims actively through the sea by
vibrating its long tail with the dorsal and ventral fins.
In addition to the structures already mentioned, the mesoderm {60}has
formed the beginning of the muscular body-wall, the connective tissue
around the organs, and the blood; the endostyle has developed as a
thick-walled groove along the ventral edge of the pharynx, which has become
the branchial sac; and the pericardial sac and its invagination the heart
have formed in the mesoblast between the endostyle and stomach. The
"epicardiac tubes" grow out from the posterior end of the endostyle to join
the pericardium. They play an important part in the formation of buds in
the colonial Tunicata. The heart acquires a connexion with blastocoelic
blood-spaces at its two ends. The heart and pericardium show the same
relations in Tunicata as in Enteropneusta, but it is very doubtful whether
these organs are genetically related to the Vertebrate heart.
[Illustration: FIG. 26.—Metamorphosis of an Ascidian. A, free-swimming
tailed larva; B, the metamorphosis—larva attached; C, tail and nervous
system of larva degenerating; D, further degeneration and metamorphosis of
larva into E, the young fixed Ascidian. _at_, Atrial invagination; _ch_,
notochord; _hy_, hypoblast cells; _i_, intestine; _m_, mouth; _mes_,
mesenteron; _n.c_, neural canal; _n.v_, neural vesicle with sense-organs.
(Modified from Kowalevsky and others.)]
The unpaired optic organ in the cerebral vesicle when fully formed has a
retina, pigment layer, lens and cornea; while the ventral median organ is a
large, spherical, partially-pigmented otolith attached by delicate
hair-like processes to the summit of a hollow "crista acustica" (Fig. 26,
A). After a few hours, or at most a day or so, the larva attaches itself
{61}by one or more of the three anterior ectodermal glandular papillae (one
dorsal and two lateral) to some foreign body, and commences the
retrogressive metamorphosis which leads to the adult state. The adhering
papillae, having performed their function, begin to atrophy, and their
place is taken by the rapidly increasing test. The tail which at first
vibrates rapidly is partly withdrawn from the test and absorbed, and partly
cast off in shreds (Fig. 26, B, C, D). The notochord, nerve-tube, muscles,
etc., are withdrawn into the body, where they break down and are absorbed
by phagocytes. The posterior part of the nerve cord and its anterior end
with the large sense-organs disappear, and the middle part or
trunk-ganglion is reduced to form the relatively small ganglion of the
adult, underneath which the hypophysial tube gives rise to the neural
gland. While the locomotory, nervous and sensory organs are thus
disappearing, or being reduced, the alimentary canal and reproductive
viscera are growing largely. The branchial sac enlarges, its walls become
penetrated by blood-channels, and grow out to form bars and papillae, and
the number of openings greatly increases by the primary gill-slits being
broken up into the transverse rows of stigmata. The stomach and intestine,
which developed as an outgrowth from the back of the branchial sac at the
right side, become longer and curve, so that the end of the intestine
acquires an opening into at first the left hand side, and eventually the
cloacal or median part of the atrial cavity. The adhering papillae have now
disappeared, and are replaced functionally by a growth of the test over
neighbouring objects; and at the same time the region of the body between
the point of fixation and the mouth (branchial aperture) increases rapidly
in extent, so as to cause the body of the Ascidian to rotate through about
180°, and thus the branchial siphon is carried to the opposite end from the
area of attachment (see Fig. 26, B, C, D, E). Finally the gonads and their
ducts form in the mesoderm between the stomach and intestine. We thus reach
the sedentary degenerate fixed adult Ascidian with little or no trace of
the Chordate characteristics so marked in the earlier larval stage (see E
and A, Fig. 26). The free-swimming tailed larva shows the Ascidian at the
highest level of its organisation, and is the stage that indicates the
genetic relationship of the Tunicata with the Vertebrata.
In some Ascidians with more food-yolk in the egg, or in which {62}the
development takes place within the body of the parent, the life-history as
given above is more or less modified and abbreviated, and in some few forms
the tailed larval stage is missing. Some exceptional cases of development
will be noted below under the groups to which they belong.
The remarkable life-history of the typical Ascidian, of which the outlines
are given above, is of importance from two points of view:—
1. It is an excellent example of degeneration. The free-swimming larva is a
more highly developed animal than the adult Ascidian. The larva is, as we
have seen, comparable with a larval fish or a young tadpole, and is thus a
Chordate animal showing evident relationship to the Vertebrata; while the
adult is in its structure non-Chordate, and is _on a level_ with some of
the worms, or with the lower Mollusca, in its organisation, although of an
entirely different type.
2. It shows us the true position of the Ascidians (Tunicata) in the animal
series. If we knew only the adult forms we might regard them as being an
aberrant group of Worms, or possibly as occupying a position between worms
and the lower Mollusca, or we might place them as an independent group; but
we should certainly have to class them as Invertebrate animals. But when we
know the whole life-history, and consider it in the light of
"recapitulation" and "evolutionary" views we recognise that the Ascidians
are evidently related to the Vertebrata, and were at one time free-swimming
Chordata occupying a position somewhere below the lowest Fishes.
{63}CHAPTER III
TUNICATA (_CONTINUED_)
CLASSIFICATION: LARVACEA—APPENDICULARIANS—STRUCTURE, ETC.—ASCIDIACEA—SIMPLE
ASCIDIANS—SPECIFIC CHARACTERS—COMPOUND ASCIDIANS—GEMMATION—MEROSOMATA—
HOLOSOMATA—PYROSOMATIDAE—THALIACEA—DOLIOLIDAE—SALPIDAE—GENERAL
CONCLUSIONS—PHYLOGENY.
We now turn to the systematic classification of the group; and further
details of structure or function, points of interest in the life-history
such as budding and the formation of colonies, the habits and occurrence,
and other peculiarities such as phosphorescence, will all be noted under
the orders, sub-orders, families and genera in which they occur.
CLASS TUNICATA.
The Tunicata or Urochordata are hermaphrodite marine Chordate animals,
which show in their development the essential Vertebrate characters, but in
which the notochord is restricted to the posterior part of the body, and is
in most cases present only during the free-swimming larval stages. The
adult animals are usually sessile and degenerate, and may be either
solitary or colonial, fixed or free. The nervous system is, in the larva,
of the elongated, tubular, dorsal, Vertebrate type, but in most cases it
degenerates in the adult to form a small ganglion placed above the pharynx.
The body is completely covered with a thick cuticular test ("tunic") which
contains a substance similar to cellulose. The alimentary canal has a
greatly enlarged respiratory pharynx or branchial sac, which is perforated
by two or many more or less modified gill-slits opening into a
peribranchial or atrial cavity, which communicates with the exterior by a
single dorsal exhalent aperture (rarely {64}two ventral apertures). The
ventral heart is simple and tubular, and periodically reverses the
direction of the blood-current.
[Illustration: FIG. 27.—Sketch of the chief kinds of Tunicata found in the
sea.]
This Class is divided into three Orders:—The Appendicularians, the
Ascidians, and the Salpians (see Fig. 27).
ORDER I. LARVACEA (APPENDICULARIANS).
Free-swimming pelagic forms, in which the posterior part of the body takes
the form of a large locomotory appendage, the {65}"tail," in which there is
a skeletal axis, the urochord. A relatively large cuticular test, the
"house," may be formed with great rapidity (in an hour or so) as a
secretion from a part of the ectoderm; it is, however, merely a temporary
structure which is soon cast off and replaced by another. The branchial sac
is simply an enlarged pharynx with two ventral ciliated openings (stigmata)
leading to the exterior. These may be regarded as the representatives of
the primary gill-slits (undivided) of the Ascidian. There are thus a single
pair. There is no separate peribranchial, atrial, or cloacal cavity. The
nervous system consists of a large dorsally placed ganglion and a long
nerve-cord, which stretches backwards over the alimentary canal to reach
the tail, along which it runs on the left side (morphological dorsal edge)
of the urochord. The anus opens ventrally on the surface of the body,
usually in front of the stigmata. No reproduction by gemmation or
metamorphosis is known in the life-history.
STRUCTURE AND MODE OF LIFE.—This is one of the most interesting groups of
the Tunicata, as it shows more completely than any of the rest the probable
characters of the ancestral forms. It has undergone little or no
degeneration, and consequently corresponds more nearly to the tailed,
larval condition than to the adult forms of the other groups. It retains,
in fact, the originally posterior, chordate, part of the body which is lost
in the metamorphosis of all the other Tunicata. Hence the Appendicularians
have been described as permanent, or sexually mature, larval forms, and
hence also the adult _Ascidia_ may be said to correspond to the trunk alone
of the Appendicularian. The Order includes a single group, the
APPENDICULARIIDA, all the members of which are minute (usually about 5 mm.
in total length) and free-swimming (Fig. 28). They occur near the surface
of the sea (and exceptionally in deeper water) in most parts of the world,
moving in a characteristic vibratory manner by the contractions of the
powerful tail (see Fig. 27). They possess the power of forming with great
rapidity, from tracts of specially large glandular ectoderm cells, the
"oikoplasts," an enormously large (many times the size of the body)
investing gelatinous layer, which probably corresponds to the test of other
groups, although it is doubtful whether it contains cellulose, and it
differs also in having no immigrated cells and in its temporary nature.
This structure (Fig. 28) was first described by Von {66}Mertens, and by him
named "Haus"; it has recently been more minutely investigated by Lohmann.
It is only loosely attached to the body, and is frequently thrown off soon
after its formation. Its function is probably protective, and possibly to
some extent hydrostatic, and it may also be of use in straining the
nutritive particles from the large volumes of water which filter through
its complicated passages and perforated folds.[98] The long, laterally
compressed "tail" in the Appendiculariida is attached to the ventral
surface of the body (Fig. 30), and is bent downwards and forwards, so that
it usually points more or less anteriorly; and is twisted through an angle
of 90°, so that the dorsal edge lies to the left. It shows what have been
interpreted as traces of metameric segmentation, having its lateral
muscle-bands broken up into successive pieces (supposed myotomes, probably
only cells), while the nerve-cord presents a series of enlargements formed
of groups of nerve-cells from which distributory nerves are given off. In
_Oikopleura_ the muscle-band in the tail is formed of ten cells fused on
each side. Near the base of the tail there is a distinctly larger elongated
ganglion. The urochord in the tail consists of a homogeneous rod surrounded
by a sheath containing nuclei.
[Illustration: FIG. 28.—Appendiculariida. A, _Appendicularia sicula_, Fol,
with house; B, _Megalocercus abyssorum_, Chun, nat. size; C, _Oikopleura
cophocerca_, Gegenb., with house; D, _Fritillaria megachile_, Fol, with
vesicle; E, Appendicularian in its house; F and G, two stages in the
formation of the house. (A to D from Seeliger; E to G from Lohmann.)]
The anterior (cerebral) ganglion has connected with it an otocyst (Fig.
29), a pigment spot, and a tubular richly ciliated process opening into the
branchial sac, and representing the dorsal tubercle and associated parts of
an ordinary Ascidian. The tube ends in a plain or coiled cellular mass
lying to the right of the ganglion. No neural gland is found.
{67}[Illustration: FIG. 29.—Transverse section through anterior part of
_Oikopleura_ to show ganglion, sense-organs, endostyle, etc. × 300. _br.s_,
Branchial sac; _c.f_, ciliated funnel; _ec_, dorsal ectoderm; _end_, closed
anterior end of endostyle; _hy_, hypobranchial groove in floor of branchial
sac; _n.g_, nerve-ganglion; _or.gl_, oral gland; _ot_, otocyst; _x_,
opening of ciliated funnel into pharynx.]
The branchial aperture or mouth leads into the simple branchial sac or
pharynx (Fig. 30, _br.s_). There are no tentacles. The endostyle is short,
is a closed tube both anteriorly and posteriorly (Fig. 29), and has about
four longitudinal rows of gland-cells. There is no dorsal lamina, and the
peripharyngeal bands run dorsally and posteriorly to unite close in front
of the oesophageal opening. The wall of the branchial sac does not show the
complex structure usual in Tunicata, and has only two ciliated apertures
(Figs. 30, 31, 32, _sg_). These are homologous with the primary stigmata of
the typical Ascidians, and with a pair of the gill-clefts of Vertebrates.
They are placed far back on the ventral surface, one on each side of the
middle line, and lead into short funnel-shaped tubes which open on the
surface of the body behind the anus (Fig. 30, _at_). These tubes correspond
to the right and left atrial involutions, which in an ordinary Ascidian
fuse to form the peribranchial cavity. The remainder of the alimentary
canal consists of oesophagus, stomach (which may have a glandular
diverticulum), intestine and rectum (Fig. 30). The heart, surrounded
ventrally by a delicate pericardial membrane, lies below and in front of
the stomach, and is formed by the differentiation of the outer ends of
epithelial cells into muscular fibrillae. Two specially large glandular
cells are placed at the opposite ends of the heart. There are no
blood-vessels except the remains of the primary body-cavity (blastocoel).
No heart can be seen in some of the smaller species of _Oikopleura_. Nearly
all the species are hermaphrodite, and the large ovary and testis are
placed at the posterior end of the body. There is no proper oviduct, the
genital {68}products merely breaking through to the exterior at the point
marked _g.d_ in Fig. 30. The spermatozoa are generally matured and shed
before the ova, and thus self-fertilisation is prevented. The ova are very
small, and little is known of the development.
[Illustration: FIG. 30.—Longitudinal optical section of _Oikopleura_. Part
of the tail is cut off. _a_, Anus; _at_, atrial opening; _br.s_, branchial
sac; _c.f_, ciliated funnel; _ec_, ectoderm; _end_, endostyle; _ep.p_,
epipharyngeal ridge; _g.d_, opening of gonads to exterior; _ht_, heart;
_hy.p_, hypopharyngeal ridge; _i_, intestine; _m_, mouth; _mus_,
muscle-bands in tail; _n_, nerve-cord; _n′_, nerve in tail; _n.ch_,
urochord; _n.g_, nerve-ganglion; _n.g′_, ganglion in tail; _oes_,
oesophagus; _or.gl_, oral gland; _ot_, otocyst; _ov_, ovary; _sg_,
stigmata; _so_, sense-organ; _sp_, testis; _st_, stomach; _t_, test. (After
Herdman.)]
CLASSIFICATION.—There are two Families of Larvacea: First, the
KOWALEVSKIIDAE, including only the remarkable genus _Kowalevskia_, Fol, in
which the heart and endostyle are absent, and the branchial sac is provided
with four rows of ciliated tooth-like processes. The two known species have
been found in the Mediterranean and in the Atlantic.
The second family APPENDICULARIIDAE comprises about eight genera, amongst
which may be mentioned:—(1) _Oikopleura_, Mertens, and (2)
_Appendicularia_, Fol, in both of which the body is short (1 or 2 mm. in
length) and compact (Fig. 30), and the tail relatively long, while the
endostyle is straight. (3) _Megalocercus_, Chun, from deep water in the
Mediterranean; _M. abyssorum_ is the largest Appendicularian known, having
a total length of {69}3 cm.—it is of a bright red colour. (4)
_Fritillaria_, Q. and G., in which the body is elongated (Fig. 32) and
composed of anterior and posterior regions, the tail relatively short, the
endostyle recurved, the stigmata opening far in front of the anus, and an
ectodermal hood is formed over the front of the body.
In all nearly forty species of Larvacea are known.
[Illustration: FIG. 31.—Transverse section of body and tail of _Oikopleura
flabellum_ (?) _at_, Atrial tube; _bl.s_, blood-space; _br.s_, cavity of
pharynx or branchial sac; _ec_, ectoderm; _en_, endoderm; _ep.p_,
epipharyngeal ciliated bands; _gel_, gelatinous layer between ectoderm and
endoderm; _hy.p_, hypopharyngeal ciliated band; _mus_, muscular tissue on
inner surface of ectoderm of tail; _n_, nerve-cord; _n′_, its continuation
in the tail; _n.ch_, notochord in tail; _r_, rectum; _sg_, one of the
stigmata or ciliated openings from the branchial sac to the atrial tube;
_t_, test (= young "house"); _x_, bridge of gelatinous tissue in front of
stigma closing branchial sac off from atrial tube. (After Herdman.)]
OCCURRENCE.—Although for the most part transparent, and usually almost
invisible in sea-water, some Appendicularians may have certain parts of the
body (alimentary canal, endostyle, gonads, etc.) brilliantly pigmented
(orange, violet, etc.), and may under exceptional circumstances be present
in such profusion as to colour tracts of the sea. Appendicularians are
widely distributed, having been found in all seas from the Arctic to the
Antarctic, both round coasts and in the open ocean. Although a few species
have been found at considerable depths in the Mediterranean, still in the
Atlantic they are not deep-water animals, and as a group must be regarded
as surface-forms. They are fairly abundant to a depth of 100 fathoms, and
some few reach 1500. Species of _Oikopleura_ and _Fritillaria_ are
{70}frequent round the British coasts, our commonest species being probably
_O. dioica_, Fol, and _F. furcata_, Moss. Young specimens appear in the
plankton about February and March, and larger forms are as a rule found
later in the summer. Several instances have been recorded of swarms of
especially large forms, provided with massive tests (the "house"), having
appeared suddenly on our coast in such abundance as to form an important
element in the surface life of the sea.
[Illustration: FIG. 32.—Diagram of _Fritillaria_ seen from the right side
to show the elongated body, the hood, and the relative positions of anus,
atrial opening, and gonads. (Compare with _Oikopleura_, Fig. 30.) _a_,
Anus; _at_, opening of atrial tube; _br.s_, branchial sac; _end_,
endostyle; _ht_, heart; _m_, mouth; _n.ch_, notochord; _n.g_,
nerve-ganglion; _oes_, oesophagus; _ov_, ovary; _sg_, stigma; _sp_, testis;
_st_, stomach.]
ORDER II. ASCIDIACEA (ASCIDIANS).
Fixed or free-swimming Simple or Compound Ascidians, which in the adult are
never provided with a locomotory appendage or tail, and have no trace of a
notochord. The free-swimming forms are colonies, the Simple Ascidians being
always sedentary and usually fixed. The test is permanent and well
developed, and becomes organised by the immigration of cells from the body;
as a rule it increases in size with the age of the individual. The
branchial sac is large and well developed. Its walls are perforated by
numerous slits (stigmata) opening into the peribranchial cavity, which
communicates with the exterior by the single atrial aperture. Many of the
Ascidiacea, both fixed and free, reproduce by gemmation to form colonies,
and in most of them the sexually produced embryo develops into a tailed
larva.
The Ascidiacea includes three groups, the Simple Ascidians, the Compound
Ascidians, and the free-swimming colonial _Pyrosoma_, which in some
respects connects this Order with the Thaliacea.
{71}SUB-ORDER 1. ASCIDIAE SIMPLICES.
Fixed Ascidians, which are solitary, and very rarely reproduce by
gemmation; if, as in a few cases, small colonies are formed, the members
are not buried in a common investing mass, but each has a distinct test of
its own. No strict line of demarcation can be drawn between the Simple and
Compound Ascidians; and one of the families of the former group, the
Clavelinidae (the "Social" Ascidians of Milne-Edwards), forms a transition
from the typical Simple forms which never reproduce by gemmation, to the
Compound forms which always do. Over 500 species of Ascidiae Simplices are
now known, but there are probably very many more still undescribed. The
sub-order may be divided into the following families:—
FAM. 1. CLAVELINIDAE.—Simple Ascidians which reproduce by gemmation to form
small colonies (Fig. 33), in which each member, or ascidiozooid, has a
distinct test, but all are connected by a common blood-system, and by a
prolongation of the "epicardiac tubes" (see p. 83) from the branchial sac.
Buds are formed on the stolons (Fig. 33), which are vascular outgrowths
from the posterior end of the body, containing prolongations from the
ectoderm, mesoderm, and endoderm (the epicardium) of the Ascidiozooid.
Branchial sac not folded; internal longitudinal bars usually absent;
stigmata straight; tentacles simple. The Clavelinidae are the simplest of
the Ascidiae Simplices. They are the forms that come nearest to the
Compound Ascidians, and are closely related to the Distomatidae. They are
probably the nearest representatives now existing of the ancestral forms
from which both Simple and Compound Ascidians are descended.
[Illustration: FIG. 33.—Colony of _Clavelina lepadiformis_ (nat. size).]
This family contains amongst others the following three
genera:—_Ecteinascidia_, Herdman, with internal longitudinal bars in the
branchial sac; _Clavelina_, Savigny, with a long body and intestine
extending behind the branchial sac (Fig. 33); and {72}_Perophora_,
Wiegmann, with a short compact body and intestine alongside the branchial
sac. _Clavelina lepadiformis_ and _Perophora listeri_ are common British
species found at a few fathoms depth off various parts of our coast. Both
occur round the south end of the Isle of Man. In autumn _Clavelina_
accumulates reserve-material in the ectoderm cells of parts of the stolon,
which remain when the rest of the colony dies away, and then form new buds
in spring.
FAM. 2. ASCIDIIDAE.—Solitary fixed Ascidians, never forming colonies; with
gelatinous or cartilaginous test; branchial aperture usually eight-lobed,
atrial aperture usually six-lobed; branchial sac not folded; internal
longitudinal bars usually present; stigmata straight or curved; tentacles
simple; gonads in or around the intestinal loop. This family is divided
into three sections:—
SUB-FAM. 1. HYPOBYTHIINAE.—Branchial sac with no internal longitudinal
bars, test strengthened with curious symmetrically placed nodules.
The one genus _Hypobythius_, Moseley, contains two stalked deep-water forms
found by the "Challenger;" _H. calycodes_ (Fig. 34, A), from the North
Pacific, 2900 fathoms, and _H. moseleyi_ from the South Atlantic, 600
fathoms.
[Illustration: FIG. 34.—A, _Hypobythius calycodes_, Moseley; B, _Chelyosoma
macleayanum_, Brod. and Sowb.; C, _Corynascidia suhmi_, Herdman; D,
_Rhodosoma callense_, Lac.-Duth.]
SUB-FAM. 2. ASCIDIINAE.—Internal longitudinal bars present; stigmata
straight. Many genera, of which the following are the more
important:—_Ciona_, Fleming, dorsal languets present; _Ascidia_, Linnaeus
(in part _Phallusia_, Savigny), dorsal lamina {73}present (Fig. 15, p. 40);
_Rhodosoma_, Ehrenberg, anterior part of test modified to form operculum
(Fig. 34, D); _Abyssascidia_, Herdman, intestine on right side of branchial
sac. The type genus of this section, _Ascidia_, has been described in
detail above (Chapter II. p. 39), and Figs. 15 to 26 illustrate its
structure and life-history. There are many species. _Ciona intestinalis_,
Linn. (Fig. 40, B), is one of the commonest of British Ascidians, and lives
readily in aquaria.
SUB-FAM. 3. CORELLINAE.—Stigmata curved and forming spirals (Fig. 35).
Three genera:—_Corella_, Alder and Hancock, test gelatinous, body sessile;
_Corynascidia_, Herdman, test gelatinous, body pedunculated (Fig. 34, C), a
remarkable deep-sea form with very delicate spirally-coiled vessels in the
branchial sac (Fig. 35, A), found in the Pacific (2160 faths.) and the
Southern Ocean; _Chelyosoma_, Brod. and Sowb., upper part of test modified
into horny plates (Fig. 34, B).
[Illustration: FIG. 35.—A, branchial sac of _Corynascidia suhmi_, Herdman;
B, branchial sac of _Corella japonica_, Herdman. _i.l_, Internal
longitudinal bars; _tr_, transverse vessels. (After Herdman.)]
_Corella_ contains several British species, one of which, _C.
parallelogramma_, O. F. Müll., is one of the commonest and most handsome
Ascidians in our coralline zone (about 20 faths.). Through its clear
crystalline test the lemon-yellow and carmine pigmentation of the mantle,
and even (with a lens) the working of the cilia along the spiral stigmata
of the branchial sac (compare Fig. 35, B), can readily be seen. The beating
of the heart can be seen just in front of the viscera upon the _right_ side
of the branchial sac (compare with _Ascidia_, Fig. 23).
In the family Ascidiidae the eggs are minute and contain {74}little or no
food-yolk, and the tailed larvae (Figs. 26, 42, A) are of the typical form
and structure described in Chapter II.
FAM. 3. CYNTHIIDAE.—Solitary fixed Ascidians (Fig. 39), sometimes occurring
in aggregations, but never forming colonies; usually with leathery or
fibrous, opaque test, which is sometimes encrusted with sand; branchial and
atrial apertures usually both four-lobed. Branchial sac longitudinally
folded (Fig. 36, A); stigmata straight; tentacles simple or compound (Fig.
37); neural gland dorsal to ganglion; gonads attached to body-wall. This
family is divided into three sections:—
[Illustration: FIG. 36.—Diagrammatic transverse sections of branchial sacs
of Cynthiidae. A, _Cynthia_; B, _Styela_; C, _Styelopsis_; D, _Pelonaia_.
_Br.f_ 1-7, First to seventh branchial fold; _d.l_, dorsal lamina; _end_,
endostyle; _mh_, meshes.]
SUB-FAM. 1. STYELINAE.—Not more than four folds (Fig. 36, B) on each side
of branchial sac; tentacles simple (Fig. 37, A). The more important genera
are—_Styela_, Macleay, and _Polycarpa_, Heller (Fig. 39), with stigmata
normal; and _Bathyoncus_, Herdman, with stigmata absent or modified. There
are a very large number of species of both _Styela_ and _Polycarpa_ from
all parts of the world, including our own seas. A very abundant British
littoral form has been placed in an allied genus under the name _Styelopsis
grossularia_ (Fig. 39, A). It is known in some places round our coasts as
"the red-currant squirter." This species has only one well-marked fold in
the branchial sac (Fig. 36, C). Another exceptional British Styelid is
_Pelonaia corrugata_, Forb. and Goods. (Fig. 39, I), with no branchial
folds (Fig. 36, D).
{75}SUB-FAM. 2. CYNTHIINAE.—More than eight folds in branchial sac (Fig.
36, A); tentacles compound (Fig. 37, B); body sessile or with a short stalk
(Fig. 39, F). The chief genus is _Cynthia_, Savigny, with a large number of
species, some of which are British. _Rhabdocynthia_ has echinated
calcareous spicules in the mantle (see Fig. 50, D, p. 87).
_Forbesella tessellata_ is a remarkable British species, having the test
marked out into plates (Fig. 39, B). It is intermediate in some characters
between Styelinae and Cynthiinae.
[Illustration: FIG. 37.—Tentacles of Cynthiidae. A, Simple, in Styelinae;
B, Compound, in Cynthiinae.]
[Illustration: FIG. 38.—_Culeolus wyville-thomsoni_, Herdman. A, from left
side (half-nat. size); B, part of branchial sac. _At_, Atrial aperture;
_Br_, branchial aperture; _br.f_, branchial fold; _i.l_, internal bar;
_sp_, spicules; _tr_, transverse vessel. (After Herdman.)]
SUB-FAM. 3. BOLTENINAE.—More than eight folds in branchial sac; tentacles
compound; body pedunculated (Fig. 38, A). The chief genera are—_Boltenia_,
Savigny, with the branchial aperture four-lobed, and the stigmata normal;
and _Culeolus_, Herdman (Fig. 38), with branchial aperture having less than
four lobes, and the stigmata absent or modified (Fig. 38, B), the branchial
sac showing a wide mesh-work of vessels stiffened by branched calcareous
spicules. _Culeolus_ is a deep-sea genus discovered by the {76}"Challenger"
expedition; eight or nine species are now known from various parts of the
world, ranging in depth from 630 to 2425 fathoms. Most of the species are
from the Pacific; only one from the North Atlantic. The curiously curved
type of spicule found in the branchial sac and other organs is shown at
Fig. 50, C (p. 87).
Amongst the Cynthiidae are found most varied conditions of the reproductive
organs. The gonads are sometimes on both, sometimes on only one side of the
body, sometimes in one or several branched masses, and sometimes
distributed as a large number of minute "polycarps" over the inner surface
of the mantle.
[Illustration: FIG. 39.—Various Cynthiidae. A, two forms of _Styelopsis
grossularia_, Van Ben.; B, _Forbesella tessellata_, Forb.; C, _Polycarpa
aurata_, Q. and G.; D, _Styela clava_, Herdman; E, _Polycarpa tinctor_, Q.
and G.; F, _Cynthia formosa_, Herdman; G, _Polycarpa comata_, Alder; H,
_Polycarpa pedata_, Herdman; I, _Pelonaia corrugata_, Forb. and Goods.
(After Herdman.)]
The family Cynthiidae is the largest section of the Simple Ascidians. The
species range from the size of a pea to that of a large cocoa-nut. They are
for the most part opaque, and often richly coloured—reds, yellows and rich
browns predominating—and so look very different to the grey gelatinous
Ascidiidae, and to the sand-encrusted Molgulidae. They extend from between
tide-marks (_Styelopsis grossularia_), down to the abysses (_Styela bythia_
and _S. squamosa_ at 2600 fathoms). Some genera (_Styela_ and the closely
related _Dendrodoa_), extend far into Arctic seas, but many allied forms
(_Styela_ and _Polycarpa_) are also found in the tropics.
{77}[Illustration: FIG. 40.—Three simple Ascidians with vascular adhering
processes from the test (nat. size). A, _Ascidiella aspersa_, O. F. Müller;
B, _Ciona intestinalis_, Linn.; C, _Molgula oculata_, Forb.]
[Illustration: FIG. 41.—Branchial sacs of Molgulidae showing curved
stigmata. A, _Ascopera gigantea_, Herdman; B, _Molgula pyriformis_,
Herdman; C, _Eugyra kerguelenensis_, Herdman.]
FAM. 4. MOLGULIDAE.—Solitary sessile Ascidians, sometimes not fixed;
branchial aperture six-lobed, atrial four-lobed. Test usually encrusted
with sand, which is generally attached to branched hair-like processes from
the test (Fig. 40, C). Branchial sac longitudinally folded; stigmata more
or less curved, usually arranged in spirals (Fig. 41); tentacles compound.
The chief genera are—_Molgula_, Forbes (Fig. 40, C), with distinct folds in
the branchial sac (Fig. 41, B), and _Eugyra_, Ald. and Hanc., with no
distinct folds, but merely broad internal longitudinal bars in the
branchial sac (Fig. 41, C). In some of the Molgulidae {78}(genus
_Anurella_, Lacaze-Duthiers), the embryo does not become converted into a
tailed larva, the development being direct without metamorphosis (see Fig.
42, C). The embryo when hatched gradually assumes the adult structure, and
never shows the features characteristic of larval Ascidians, such as the
urochord and the median sense-organs. Fig. 42 shows an Ascidiid (A), a
Cynthiid (B), and this exceptional Molgulid (C), type of larva, and three
forms of Compound Ascidian larvae, the Distomatid (D), the Botryllid (E),
and the Diplosomatid (F).
[Illustration: FIG. 42.—Larvae of various Ascidians. A, _Ascidia mentula_,
Linn.; B, _Polycarpa glomerata_, Alder; C, _Anurella roscovita_,
Lac.-Duth.; D, _Distaplia magnilarva_, Della Valle; E, _Polycyclus
renieri_, Lamk.; F, _Diplosomoides lacazii_, Giard. (Mostly after
Lahille.)]
In the Molgulidae the viscera are characteristic in position and
appearance. The alimentary canal lies on the left side of the branchial
sac, and the intestine forms a long narrow loop directed in the main
transversely. The pericardium and heart are on the middle of the right
side, and behind them is placed the single sac-like ductless renal organ,
generally occupied by one or more concretions. The gonads are in most cases
on both sides of the body, in front of the intestine on the left, and in
front of the heart on the right; but in _Eugyra_ there is no gonad on the
right side, and in some other forms the gonad on the left side is absent.
(For _Oligotrema_, see p. 111, note.)
There are a number of British Molgulidae, the two commonest {79}of which
are—_Molgula oculata_, Forbes, thickly covered with gravel or broken
shells, and forming an ovate mass as large as a walnut; and _Eugyra
glutinans_, Möller, a smaller more globular body, the size of an acorn, and
covered with fine sand, except at one circular area near the posterior end,
where the leaden grey test shows through. Both these species are obtained
by dredging in from 10 to 30 fathoms, and lie freely on the bottom. A
rather rarer littoral species _Molgula citrina_, Hancock, found on some
parts of our coast (_e.g._ in the Firth of Forth, at Arran, and at Port
Erin), is exceptional in having the test free from sand, and in being fixed
like an _Ascidia_, generally to the lower surfaces of large stones near low
tide.
SPECIFIC CHARACTERS AND DORSAL TUBERCLE.—The chief points in which the
various genera and species of Simple Ascidians differ are the details of
the branchial sac (see Figs. 22, 35, 36, 38, and 41), the condition of the
tentacles (Fig. 37), the dorsal lamina or languets, and the dorsal
tubercle, in addition to form, colour, and other external features.
[Illustration: FIG. 43.—Various forms of dorsal tubercle in Simple
Ascidians. 1. _Molgula pyriformis_; 2. _Forbesella tessellata_; 3. _Ascidia
meridionalis_; 4. _Cynthia formosa_; 5. _Cynthia papietensis_; 6. _Ascidia
challengeri_; 7. _Polycarpa tinctor_; 8. _Cynthia cerebriformis_;
9. _Ascopera gigantea_; 10. _Boltenia tuberculata_; 11. _Ascidia
translucida_; 12. _Culeolus moseleyi_; 13. _Ascidia pyriformis_;
14. _Boltenia pachydermatina_; 15. _Microcosmus draschii_; 16. _Styela
etheridgii_; 17. _Styela whiteleggii_; 18. _Polycarpa aurata_. (After
Herdman.)]
Fig. 43 shows some of the more remarkable forms of dorsal tubercle.
Starting with a simple circular opening (1) surrounded by a thickened
ciliated ring, the anterior border becomes pushed in to form a crescentic
slit (2 and 3). The horns of the crescent then grow longer and may be
turned in (4 and 5) or out (6 and 7), and so give rise to the many
varieties of horse-shoe (such as 6), perhaps the commonest form of dorsal
tubercle in Simple Ascidians. In many Cynthiidae the central part of the
{80}horse-shoe remains small, while the horns become long and much coiled
so as to constitute two prominent spirals (8, 9, 10). In other exceptional
forms again the curved slit becomes straightened out, undulating (11),
irregularly bent (12 and 13), elaborately folded (14 and 15), or broken up
into pieces (16), so that there come to be several or even a large number
(17 and 18) of minute openings in place of the original single aperture.
It cannot be said that any form of dorsal tubercle is characteristic of any
of the families or genera of Ascidians, and in the case of some species the
organ is liable to great individual variation; but still in most species
there is found to be a characteristic shape or appearance of tubercle which
is a useful diagnostic feature.
SUB-ORDER 2. ASCIDIAE COMPOSITAE.
Fixed Ascidians which reproduce by gemmation so as to form colonies (Fig.
44) in which the ascidiozooids are buried in a common investing mass (Fig.
45) and have no separate tests—hence "Synascidiae," a name they often
receive from foreign writers.
[Illustration: FIG. 44.—Colonies of Compound Ascidians (nat. size). A,
_Colella quoyi_, Hrdn. Antarct.; B, _Leptoclinum neglectum_, Hrdn.; C,
_Pharyngodictyon mirabile_, Hrdn. Southern Ocean; D, _Botryllus
schlosseri_, Sav. Europe. (After Herdman.)]
This is probably a somewhat artificial assemblage formed of those two or
three groups of Ascidians which produce colonies, in which the
ascidiozooids are so intimately united that they possess a common test or
investing mass. This is the only character which distinguishes them from
the Clavelinidae, but the property of reproducing by gemmation separates
them from the rest of the Ascidiae Simplices. In some cases the atrial
apertures of several neighbouring ascidiozooids join to open to the
exterior by a common cloacal aperture (Fig. 45, _c.c_). Such {81}groups of
the ascidiozooids of a colony are known as "systems" or coenobia (see Fig.
44, D; also Fig. 53, p. 89).
The Ascidiae Compositae may be divided into seven families, which seem to
fall into two well-marked sets:—(1) MEROSOMATA, in which the heart and
alimentary and reproductive viscera are placed behind the branchial sac, so
as to constitute a more or less extended body divided into at least two
regions (Fig. 46, B), and sometimes three (Fig. 46, C)—thorax, abdomen, and
post-abdomen; and (2) HOLOSOMATA, in which the body of the ascidiozooid is
short, compact, and not divided into regions (Fig. 46, A). The latter group
comprises the two families Botryllidae and Polystyelidae, which agree both
in points of structure and in having the same type of budding, and are
probably derived from ancestral Cynthiidae amongst Simple Ascidians; while
the Merosomata seem more nearly related to the Clavelinidae.
[Illustration: FIG. 45.—Vertical section through a small part of a compound
Ascidian colony. _Asc_. 1 and _Asc_. 2, Parts of two ascidiozooids whose
cloacas (_cl_) open into the common cloacal cavity (_c.c_) of the colony;
_at.l_, atrial lobes; _t_, _t_, _t_, common test of the colony. The
structure of the posterior parts of the ascidiozooids would depend upon the
family (see Fig. 46). The arrows show the direction of the water currents.]
GEMMATION takes place in the Compound Ascidians in a variety of ways, being
sometimes very different in its details in closely allied forms. There are,
however, two main types of budding, to one or other of which most of the
described methods may be referred. These are:—
1. The STOLONIAL, or "epicardiac" type—seen in the Merosomata, typically in
Distomatidae and Polyclinidae, and comparable with the gemmation in
Clavelinidae, Pyrosomatidae, and Thaliacea outside this group.
{82}2. The PARIETAL, or "peribranchial" type—seen in the Holosomata,
typically in the Botryllidae.
The remarkable process of gemmation seen in the families Didemnidae and
Diplosomatidae, where the bud arises from at least two rudiments, the one
stolonial or epicardiac in origin, and the other formed by one or more
oesophageal or intestinal outgrowths, has been called "entero-epicardiac,"
but it may probably be regarded as a modification of the stolonial type.
[Illustration: FIG. 46.—A, Ascidiozooid from a Botryllid colony; B,
ascidiozooid from a Distomid colony; C, ascidiozooid from a Polyclinid
colony. _a_, Anus; _at_, atrial aperture; _at.l_, atrial languet; _br_,
branchial aperture; _cl_, cloaca; _d.l_, dorsal languet; _ec_, ectoderm;
_end_, endostyle; _ep.c_, epicardiac tube; _gl_, intestinal gland; _h_,
heart; _i_, intestine; _n.g_, nerve-ganglion; _oes_, oesophagus; _ov_,
ovary; _p.c_, pericardium; _r_, rectum; _sg_, stigmata of branchial sac;
_sp_, spermatic sacs; _sph_, sphincter; _st_, stomach; _t_, tentacle;
_t.k_, terminal ampullae of vessels in test; _v_, colonial vessels;
_v.app_, "vascular appendage" (stolon).]
The marked differences in the appearance of the colonies of Compound
Ascidians is largely due to the methods of budding; and even in those of
the stolonial type, where the budding is practically the same in essential
nature, the results may be very different in superficial appearance,
according as the buds are {83}formed on a short stolon close to the parent
body, or from the extremity of the post-abdomen (as in the Polyclinidae),
or from a long epicardiac tube (as in _Colella_, Fig. 47), which may extend
for some inches from the ascidiozooid. The post-abdomen of the Polyclinidae
may be regarded as a stolon invaded by the gonads and the heart (see Fig.
46, C), and traversed by the epicardium in the form of a flattened tube
dividing a dorsal blood-sinus containing the gonads from a ventral sinus
which has merely the one extremity of the tapering pericardium. The whole
of this post-abdomen segments to form the buds, the heart at the extremity
being absorbed, and a new one formed from the anterior end of the
pericardium.
The epicardium, which supplies the endodermal element to each bud, was
first described by E. van Beneden and Julin in the envelopment of
_Clavelina_,[99] as a structure concerned in the formation of the
pericardium and heart—hence its unfortunate name. It grows backwards in the
larva, from the posterior wall of the branchial sac, close to the
endostyle, as a tube which usually divides into two lateral branches to be
united again eventually so as to form the single tubular flattened
partition of the stolon in Polyclinidae, Distomatidae, Clavelinidae, etc.
In some Compound Ascidians the epicardium is, from its origin, two distinct
lateral tubes, which grow back from the inner vesicle of the embryo (later
the branchial sac). These unite in the post-abdomen to form the flattened
tube, which in its turn forms the inner vesicle of the future buds, and so
the endodermal element is handed on from generation to generation. In
addition to the epicardium, the stolon contains also a prolongation of the
ovary of the parent, or at least a string of migrating germ cells, so that
the reproductive elements are also handed on.
It is clear from the recent researches of Hjort, Ritter, Lefevre,[100] and
others, that the development of the bud (blastozooid) and that of the
embryo (oozooid) do not proceed along parallel lines. It is evidently
impossible to harmonise the facts of gemmation with the germ-layer theory;
and attempts to explain budding in Ascidians solely as a process of
regeneration by which the organs of the parent or their germ-layers give
rise to the corresponding organs in the bud have in many cases failed.
{84}The rudiment of the bud is in typical cases composed of two vesicles,
an outer derived from the ectoderm of the parent and enclosing free
blood-cells (mesodermal) between its wall and that of the inner
vesicle—which is usually of endodermal origin, but in Botryllidae is
derived from the peribranchial sac, an ectodermal structure. The inner
vesicle, derived in the two cases from different germ-layers, forms the
same organs of the bud, and these organs may be of widely different origin
in the larva. Moreover, free cells of the blood may play in the bud a very
important part, and give rise (_Perophora_) to such important systems as
pericardium and heart, neural tube and ganglion, the gonads and their
ducts, some of which are of ectodermal and others of endodermal origin in
the larva.
In some cases of precocious budding (blastogenetic acceleration) the young
buds begin to appear during the tailed larval stage. The larva may even
contain a first blastozooid (bud) with a branchial sac as large as that of
the oozooid (derived from the egg); and in the Diplosomatidae the larva
(see Fig. 42, F), when it settles down, may be already a small colony of
three young ascidiozooids.
The larvae in most Compound Ascidians, in place of adhering papillae, have
several or even a considerable number of ectodermal tubes or prolongations
from the body (see Fig. 42, E and F) into the surrounding test. These
apparently aid in the formation of the common test of the young colony,
which grows over and adheres to foreign objects.
There are many irregularities in the larval development of Compound
Ascidians, due to the very different amount of food-yolk present in the ova
in different genera. In some cases there is even dimorphism, two forms of
larvae being found in the same colony.
Compound Ascidians are amongst the most varied and brilliant of sessile
animals seen at low tide on our own and most other coasts. Some are stalked
and form club-shaped or knob-like outgrowths. Others again form flat
gelatinous expansions attached to sea-weeds or stones, and are
symmetrically marked with bright spots of colour in the form of circles,
meandering lines, or star-like patterns. In such colonies each spot of
colour or ray of a star represents an ascidiozooid or member of the colony,
equivalent to the whole animal in the case of the solitary Simple Ascidian.
{85}GROUP A. _MEROSOMATA_.
Viscera posterior to branchial sac; budding stolonial.
[Illustration: FIG. 47.—A, Colony of _Colella pedunculata_, Q. and G., nat.
size: _a_, zone of buds; _b_, zone of young ascidiozooids; _c_, zone of
reproducing adults; _d_, old decaying adults and incubatory pouches with
larvae. B, Ascidiozooid, with incubatory pouch enlarged: _At_, atrial
aperture; _Br_, branchial aperture; _emb_, embryos; _end_, endostyle;
_ep.c_, epicardium; _inc.p_, incubatory pouch; _od_, oviduct; _od′_, its
prolongation into _inc.p_; _od″_ its termination at tip of _inc.p_; _ov_,
ovary; _p.br_, peribranchial opening of _inc.p_; _st_, stomach.]
FAM. 1. DISTOMATIDAE.—Ascidiozooids divided into two regions, a thorax,
containing the branchial sac, and an abdomen, with the remaining viscera
(Fig. 47, B); testes numerous; vas deferens not spirally coiled. The chief
genera are—_Distoma_, Gaertner, with some British species;
_Chondrostachys_, Macdonald, _Cystodytes_, v. Drasche, with calcareous
plate-like spicules in the test (Fig. 50, A); _Distaplia_, Della Valle, and
_Colella_, Herdman, forming a pedunculated colony (Fig. 47, A), in which
the ascidiozooids (Fig. 47, B) are provided with large incubatory pouches,
opening from the peribranchial cavity, but also connected, as Bancroft[101]
has recently shown, with the end of the oviduct (see Fig. 47, B). In these
pouches the embryos undergo their development, and are set free by the
decay of the top of the colony. The stolons pass from the ascidiozooids in
the upper part of the colony down into the stalk, and there produce buds
which gradually work up to the top of the stalk, where they take their
places as young ascidiozooids. At the top of the colony the old
ascidiozooids die and are removed (see Fig. 47, A). Caullery has shown that
in {86}this genus there may be dimorphism in the buds, some of them placed
deeply in the stalk having a large amount of reserve food-matter in their
ectoderm, and remaining dormant until required to regenerate the "head" or
upper part of the colony when it is lost. This genus was made known by the
"Challenger" expedition. The species are mostly tropical, or from southern
seas.
FAM. 2. COELOCORMIDAE.—Colony not fixed, having a large axial cavity with a
terminal aperture. Branchial apertures five-lobed. This includes one
species, _Coelocormus huxleyi_, Herdman, which is in some respects a
transition-form between the ordinary Compound Ascidians (_e.g._
Distomatidae) and the Ascidiae Luciae (_Pyrosoma_, see p. 90).
[Illustration: FIG. 48.—Transverse section of the abdomen of a Distomid.
_bl.s_, Blood-sinus; _ec_, ectoderm; _ep.c_, epicardium; _gl_, intestinal
glands; _h_, heart; _i_, intestine; _l.m_, longitudinal muscles; _mes_,
mesoderm; _o.d_, oviduct; _p.c_, pericardium; _st_, stomach; _v.d_, vas
deferens. (After Seeliger.)]
[Illustration: FIG. 49.—Section of _Leptoclinum_ colony, showing the
distribution of spicules and parts of the ascidiozooids. _b_, Base of
colony; _br_, branchial aperture; _br.s_, branchial sac; _sp_, spicules;
_st_, stomach; _tes_, testis; _v.d_, vas deferens.]
FAM. 3. DIDEMNIDAE.—Colony usually thin and incrusting. Test containing
stellate calcareous spicules (Figs. 49 and 50, B). {87}Testis single,
large; vas deferens spirally coiled (Fig. 49). The chief genera
are—_Didemnum_, Savigny, in which the colony is thick and fleshy, and there
are only three rows of stigmata on each side of the branchial sac; and
_Leptoclinum_, Milne-Edwards, in which the colony is thin and incrusting
(Fig. 49), and there are four rows of stigmata. Colonies of _Leptoclinum_,
forming thin white, grey, or yellow crusts under stones at low water, are
amongst the commonest of British Compound Ascidians.
[Illustration: FIG. 50.—Calcareous spicules of the Tunicata, enlarged. A,
From _Cystodytes_; B, from _Leptoclinum_; C, from _Culeolus_; D, from
_Rhabdocynthia_.]
FAM. 4. DIPLOSOMATIDAE.—Test reduced in amount (Fig. 51), rarely containing
spicules. Vas deferens not spirally coiled. In _Diplosoma_, Macdonald, and
other allied genera (Fig. 51), the larva is gemmiparous (Fig. 42, F). Some
species are common British forms, especially on _Zostera_-beds and amongst
seaweeds.
[Illustration: FIG. 51.—Section of a colony of _Diplosoma_ (enlarged) to
show the small amount of test present. _br_, Branchial aperture; _c.cl_,
common cloaca; _t_, test.]
FAM. 5. POLYCLINIDAE.—Ascidiozooids divided into three regions—thorax,
abdomen, and post-abdomen (Fig. 46, C). Testes numerous; vas deferens not
spirally coiled. The chief genera are—_Pharyngodictyon_, Herdman, with
stigmata absent or modified, containing one species, _Ph. mirabile_ (Fig.
44, C), the {88}only Compound Ascidian known from a depth of 1000 fathoms;
_Polyclinum_, Savigny, with a smooth-walled stomach (Fig. 52, A);
_Aplidium_, Savigny, with the stomach-wall longitudinally folded (Fig. 52,
B); _Morchellium_, Giard, with an "areolated" stomach (Fig. 52, D), bearing
knobs on the outside; and _Amaroucium_, Milne-Edwards, in which the
ascidiozooid has a long post-abdomen and a large atrial languet, and where
the stomach-wall shows longitudinal ridges breaking up into knobs
(pseudo-areolated, Fig. 52, C). The last four genera contain many common
British species.
[Illustration: FIG. 52.—Various conditions of stomach in Polyclinidae. A,
_Polyclinum molle_, Herdman; B, _Aplidium zostericola_, Giard; C,
_Amaroucium proliferum_, M.-Edw.; D, _Morchellium argus_, M.-Edw.]
Many of the Compound Ascidians die down in winter; but amongst
Polyclinidae, as in _Clavelina_, a form of hibernation is found, the old
ascidiozooids dying, but some of the buds in the basal part of the colony
accumulating a large store of reserve-material in their ectoderm, and lying
dormant until spring, when they regenerate the colony.
GROUP B. _HOLOSOMATA_.
Body short, compact, with viscera by the side of branchial sac; budding
parietal
FAM. 6. BOTRYLLIDAE.—Ascidiozooids grouped in systems round common cloacal
apertures (Fig. 53). Ascidiozooids having the intestine and reproductive
organs by the side of the branchial sac (Fig. 46, A, p. 82). Dorsal lamina
and internal longitudinal bars present in the branchial sac. Neural gland,
as in Cynthiidae, dorsal to the ganglion in place of ventral as in the
majority of Tunicata. The chief genera are—_Botryllus_, Gaertn. and Pall,
with simple stellate systems (Fig. 53), and _Botrylloides_, Milne-Edwards,
with elongated or ramified systems. There are {89}many species of both
these genera, which form brilliantly coloured fleshy crusts under stones
and on sea-weeds at low tide. They are amongst the commonest and the most
beautiful of British Ascidians. Both genera contain species remarkable for
the rich profusion of ectodermal "vessels" which ramify and anastomose in
the colonial test. On the margins of the colony these vessels end in
knob-like dilatations, the ampullae (Fig. 46, A, _t.k_), which are said by
Bancroft to pulsate rhythmically, and so aid in keeping up the colonial
circulation. They are also storage reservoirs for the blood, doubtless help
in respiration, and are organs for the secretion of the test-matrix.
[Illustration: FIG. 53.—Two "systems" from a colony of _Botryllus
violaceus_, M.-Edw. _cl_, Common cloaca of a system; _or_, branchial
apertures of ascidiozooids, magnified. (After H. Milne-Edwards.)
FIG. 54.—_Goodsiria placenta_, Herdman. A, Colony (half nat. size); B,
section of colony showing ascidiozooids. (After Herdman, from _Challenger
Reports_.)]
FAM. 7. POLYSTYELIDAE.—Ascidiozooids not grouped in systems; branchial and
atrial apertures four-lobed; branchial sac may be folded; internal
longitudinal bars present. The chief genera are—_Thylacium_, Carus, with
the ascidiozooids projecting above the general surface of the colony;
_Goodsiria_, Cunningham, with the ascidiozooids completely imbedded in the
investing mass (Fig. 54); and _Chorizocormus_, Herdman, with the
ascidiozooids {90}united in little groups which are connected by stolons.
The last genus contains one species, _Ch. reticulatus_, in some respects a
transition-form between the other Polystyelidae and the Styelinae amongst
Simple Ascidians.
BUDDING IN HOLOSOMATA—In the Polystyelidae, according to Ritter,[102] the
budding is of the same type as in Botryllidae, the bud arising in each case
from the lateral body-wall of the parent.
In _Botryllus_[103] the oozooid formed from the larva gives rise at a very
early period to the first blastozooid of the future colony. This then forms
the two buds of the second generation on its sides (see Fig. 55), and these
in their turn form the third, and these the fourth generation, in which
there are thus eight blastozooids; and so the process goes on, the buds of
each generation arranging themselves in a circle to form a system. As each
new generation makes its appearance, the preceding one undergoes
degeneration, and is eventually absorbed. Consequently, in a system there
can usually be seen, in addition to the adult members, certain older ones
in various stages of degeneration and removal, and certain younger ones
arising as buds on the sides of their predecessors, or just separated from
them, and ready to take their places as young ascidiozooids in the system.
Three distinct generations are thus commonly seen in a system. Now and
again one or two young ascidiozooids become squeezed by the pressure of
their neighbours out of a system into the surrounding test, and so give
rise to new systems which add to the extent of the colony.
[Illustration: FIG. 55.—Diagram to illustrate the budding and formation of
a system in _Botryllus_. _Ooz_; oozooid; _Bl_ 1, first blastozooid; 2, 2,
etc., successive generations of buds.]
SUB-ORDER 3. ASCIDIAE LUCIAE.
Free-swimming pelagic colonies having the form of a hollow cylinder closed
at one end (Fig. 56). The ascidiozooids forming the colony are imbedded in
the common test in such a manner that the branchial apertures open on the
outer surface and the {91}atrial apertures on the inner surface next to the
central cavity of the colony. They are placed with their ventral surfaces
towards the closed end (Fig. 56, C). The first ascidiozooids of a colony
are produced by gemmation from a stolonic prolongation of an imperfect
oozooid or rudimentary larva (the "cyathozooid"), developed sexually. The
subsequent ascidiozooids are formed from these as buds on a ventral stolon.
This sub-order includes a single family, the PYROSOMATIDAE, containing one
well-marked genus _Pyrosoma_, Péron, with about six species. They are found
swimming near the surface of the sea, chiefly in tropical latitudes, and
are brilliantly phosphorescent. A fully developed _Pyrosoma_ colony may be
from an inch or two to upwards of twelve feet in length.
[Illustration: FIG. 56.—_Pyrosoma_. A, lateral view (nat. size); B, end
view; C, diagram of longitudinal section. _at_, Atrial apertures; _br_,
branchial apertures; _c.cl_, common cloaca; _end_, endostyle; _t_, test;
_v_, velum or diaphragm at terminal opening.]
THE COLONY.—The shape of the colony is seen in Fig. 56, A. It tapers
slightly towards the closed end, which is rounded. The opening at the
opposite end may be reduced in size (see B and C), by the presence of a
membranous prolongation of the common test, which can be contracted or
expanded by means of the muscle-bands it receives from the atrial siphons
of neighbouring zooids. The branchial apertures of the ascidiozooids are
mostly placed upon short (in some cases longer) papillae projecting from
the general surface, and many of the ascidiozooids have long conical
processes of the test extending outwards beyond their branchial
{92}apertures (Fig. 57, _t′_). There is only a single layer of adult
ascidiozooids in the wall of the _Pyrosoma_ colony, as all the fully
developed ascidiozooids are placed with their antero-posterior axes at
right angles to the surface and communicate by their atrial apertures with
the central cavity (Fig. 56, C). Their dorsal surfaces are turned towards
the open end of the colony, and the buds are given off from their ventral
edges (Fig. 57).
[Illustration: FIG. 57.—Ascidiozooid of _Pyrosoma_ from the right side.
_a_, Anus; _At_, atrial aperture; _at.m_, atrial muscles; _Br_, branchial
aperture; _br.s_, branchial sac; _cl_, cloaca; _d.l_, dorsal lamina; _d.t_,
dorsal tubercle; _ec_, ectoderm; _en_, endoderm; _end_, endostyle; _Ht_,
heart; _l.o_, luminous organ; _mes_, mass of mesoderm cells; _m.f_, muscle
fibre; _n.g_, nerve-ganglion; _oes_, oesophagus; _sg_, stigmata; _st_,
stomach; _stol_, stolon; _t_, test; _t′_, projection of test near branchial
aperture; _tes_, testis; _tn_, tentacle; 1, 2, 3, buds.]
ANATOMY.—The more important points in the structure of the ascidiozooid of
_Pyrosoma_ are shown in Fig. 57. A circle of tentacles, of which one,
placed ventrally (_tn_), is larger than the rest, is found just inside the
circular branchial aperture. From this point a wide cavity, with a few
circularly placed muscle-bands running round its walls, leads back to the
large branchial sac (_br.s._), which occupies the greater part of the body.
The large stigmata are elongated transversely (dorso-ventrally), and are
crossed by internal longitudinal bars running antero-posteriorly. The
dorsal lamina is represented by a series of eight or ten {93}languets. The
nerve-ganglion (on which is placed a small pigmented sense-organ, the
unpaired "eye"), the neural gland, the dorsal tubercle, the peripharyngeal
bands and the endostyle are placed in the usual positions. On each side of
the anterior end of the branchial sac, close to the peripharyngeal bands is
a mass of rounded mesodermal gland-cells (_l.o_), which are the source of
the phosphorescence. They are apparently modified leucocytes lying in
blood-sinuses. The alimentary canal is placed posteriorly to the branchial
sac, and the anus opens into a large peribranchial or atrial cavity, of
which only the median posterior part (_cl_), is shown in Fig. 57. The heart
(_Ht_) lies between the posterior end of the branchial sac and the
intestine, close to where the endostyle is prolonged outwards to form the
inner tube of the ventral stolon. The reproductive organs are developed
from a cord of germinal tissue which forms a part of every budding stolon,
and so establishes a continuity of origin between the ova of successive
generations of _Pyrosoma_. On the ventral edge of the body, immediately
behind the stolon, with part of which it is continuous, a portion of this
germinal tissue gives rise to a lobed testis (_tes_), and to a single ovum
surrounded by indifferent or follicle-cells.
DEVELOPMENT AND LIFE-HISTORY.—The development takes place within the body
of the parent, in a part of the peribranchial cavity. It is a "direct"
development, the tailed larval stage being omitted. The segmentation is
incomplete or "meroblastic," and an elongated embryo is formed on the
surface of a mass of food-yolk. Follicle-cells, or kalymmocytes, migrate
into the embryo, where they aid in its nutrition. The embryo (or young
oozooid),[104] after the formation of an alimentary cavity, a tubular
nervous system, and a pair of laterally placed atrial tubes, divides into
an anterior and a posterior part (see Fig. 58). The anterior and ventral
part, or stolon, then segments into four pieces (the tetrazooids or first
blastozooids),[104] which afterwards develop into the first ascidiozooids
of the colony, while the posterior part remains in a rudimentary condition,
and is what was called by Huxley the "cyathozooid" (Fig. 58, _cy_). This is
really the degenerate oozooid, and eventually atrophies without having
{94}completed its development, but having precociously given rise to the
budding stolon.
As the four ascidiozooids increase in size, they grow round the cyathozooid
and soon encircle it (Fig. 58, B). In this condition the young colony
leaves the body of the parent and becomes free. The cyathozooid absorbs the
nourishing yolk upon which it lies, and distributes it to the ascidiozooids
by means of a heart and system of vessels which have been meanwhile formed.
When the cyathozooid atrophies and is absorbed, its original atrial
aperture remains and deepens to become the central cavity[105] of the young
colony, which now consists of four ascidiozooids placed in a ring, around
where the cyathozooid was, and enveloped in a common test. The test is at
first formed by the ectoderm cells of the cyathozooid. Later it becomes
invaded by mesoblast cells from the ascidiozooids in the usual manner. The
colony gradually increases by the formation of buds from these four
original ascidiozooids. The young colony is, in some species, at first
male, and only becomes hermaphrodite when it has attained to some size.
[Illustration: FIG. 58.—Development of _Pyrosoma_ colony. A, young stage
showing oozooid or cyathozooid, _cy_, with stolon divided into four
blastozooids (I.-IV.): _v_, vitellus. B, older stage showing the four
blastozooids in a ring around the remains of the cyathozooid. (After
Salensky.)]
OCCURRENCE.—The half-dozen known species of _Pyrosoma_ are widely
distributed over the great oceans, although they are probably most abundant
in tropical waters. _Pyrosoma atlanticum_, Péron, and _P. giganteum_,
Lesueur, are the commonest forms. Although sometimes abundant in the
Mediterranean and the North Atlantic they have apparently not been found in
British seas. _P. elegans_, Lesueur, is a Mediterranean form allied to the
last two; and _P. minatum_ and _P. aherniosum_, Seeliger, were discovered
during the German "Plankton" expedition in the tropical Atlantic. Finally,
the enormous _P. spinosum_, Herdman, was found by the "Challenger" in both
North and South Atlantic in 1873; and {95}some years later (Perrier's _P.
excelsior_) by the French "Talisman" expedition in the tropical Atlantic.
The late Professor Moseley said of this ("Challenger") species, "I wrote my
name with my finger on the surface of the giant _Pyrosoma_ as it lay on
deck in a tub at night, and my name came out in a few seconds in letters of
fire." Bonnier and Pérez have recently recorded that they saw an enormous
profusion of a large _Pyrosoma_ (up to four metres in length) in the
Arabian part of the Indian Ocean.
ORDER III. THALIACEA (SALPIANS).
Free-swimming pelagic forms of moderate size, which may be either simple or
compound, and in which the adult is never provided with a tail or
notochord. Consequently the whole body here corresponds to the trunk only
of the Appendicularian without the tail. The test is permanent, and may be
either well developed or very slight. In all cases it is clear and
transparent. The musculature of the body-wall is in the form of more or
less complete circular bands, by the contraction of which water is ejected
from the body, and so locomotion is effected. The branchial sac has either
two large, or many small, stigmata, leading to a single peribranchial
cavity, into which the anus also opens. Blastogenesis takes place from a
ventral, endostylar stolon. Alternation of generations occurs in the
life-history, and may be complicated by polymorphism. The Order Thaliacea
comprises two groups, CYCLOMYARIA (such as _Doliolum_) and HEMIMYARIA (such
as _Salpa_).
SUB-ORDER 1. CYCLOMYARIA.
Free-swimming pelagic forms which exhibit alternation of generations in
their life-history, but never form permanent colonies. The body is
cask-shaped, with the branchial and atrial apertures at the opposite ends.
The test is moderately well developed, never much thickened. The
musculature is mostly in the form of complete circular bands surrounding
the body. The branchial sac is fairly large, occupying the anterior half or
more of the body. Stigmata are usually present in its posterior part only.
The peribranchial cavity is mainly posterior to the branchial sac. The
alimentary canal is placed ventrally, close to {96}the posterior end of the
branchial sac. Hermaphrodite reproductive organs lie ventrally near the
intestine.
This group is clearly distinguished from the second sub-order, the
Hemimyaria, by the condition of the muscle-bands and of the branchial sac,
and by the life-history. The muscle-bands are complete rings (except in
_Anchinia_), while in the Hemimyaria they are always more or less
incomplete. The branchial sac in the Cyclomyaria is a distinct cavity, and
communicates with the peribranchial cavity only by small slits or stigmata.
The life-history is also very characteristic, as the sexual generation in
the Cyclomyaria is always polymorphic, while in the Hemimyaria it consists
of one form only.
[Illustration: FIG. 59.—Sexual generation of _Doliolum tritonis_, Herdman,
from left side, × 10. _at_, Atrial aperture; _at.l_, atrial lobes; _at.m_,
wall of atrium; _br_, branchial aperture; _br.l_, branchial lobes; _br.s_,
branchial sac; _d.t_, dorsal tubercle; _end_, endostyle; _h_, heart; _i_,
intestine; _m_, mantle; _m_^1-_m_^8, circular muscle-bands; _n_, nerve;
_n.g_, nerve-ganglion; _ov_, ovary; _p.br_, peribranchial cavity; _p.p_,
peripharyngeal bands; _sg_, stigmata; _s.gl_, neural gland; _s.o_,
sense-organ; _st_, stomach; _t_, test; _tes_, testis; _z_, prebranchial
zone. (After Herdman.)]
STRUCTURE OF DOLIOLUM.—The single family DOLIOLIDAE includes three genera,
_Doliolum_, Quoy and Gaimard, _Dolchinia_, Korotneff, and _Anchinia_,
Eschscholtz. _Doliolum_, of which about a dozen species are known, from
various seas, has a cask-shaped body (Fig. 59), usually from 1 to 2 cm. in
length. The terminal branchial and atrial apertures are lobed, and the
lobes are provided with sense-organs. The test is a thin but tough
transparent layer, and contains no "test" cells. It is merely a cuticle
covering the surface of the squamous ectoderm. The body-wall has eight or
nine circular muscle-bands surrounding the body. The most anterior and
posterior of these form the branchial and atrial sphincters. The wide
branchial and atrial apertures lead respectively into branchial and
peribranchial cavities separated by the posterior and postero-lateral walls
of the branchial sac which are pierced by a considerable number of
{97}small stigmata; consequently there is a free passage for the water
through the body along its long axis, and the animal swims by contracting
its ring-like muscle-bands so as to force out the contained water
posteriorly. When stigmata are found on the lateral walls of the branchial
sac (see Fig. 59) there are corresponding anteriorly directed diverticula
of the peribranchial cavity. There is a distinct endostyle on the ventral
edge of the branchial sac and a peripharyngeal band surrounding its
anterior end, but there is no representative of the dorsal lamina along its
dorsal edge; and there are neither branchial nor atrial tentacles. The
oesophagus commences rather on the ventral edge of the posterior end of the
branchial sac, and runs backwards to open into the stomach, which is
followed by a curved intestine opening into the peribranchial cavity. The
alimentary canal as a whole is to the right of the middle line. The
hermaphrodite reproductive organs are to the left of the middle line
alongside the alimentary canal. They open into the peribranchial cavity.
The ovary is nearly spherical, while the testis is elongated, and may be
continued anteriorly for a long distance. The heart is placed in the middle
line ventrally, between the posterior end of the endostyle and the
oesophageal aperture. The nerve-ganglion lies about the middle of the
dorsal edge of the body, and gives off many nerves. Under it is placed the
neural gland, the duct of which runs forward and opens into the anterior
end of the branchial sac by a simple aperture surrounded by the spirally
twisted dorsal ends of the peripharyngeal bands.
LIFE-HISTORY.—The ova produced by the _Doliolum_ of the sexual generation,
after a complete or "holoblastic" segmentation, and normal invagination,
produce tailed larvae with a relatively small caudal appendage, and a large
body in which the characteristic musculature begins to appear (Fig. 60, A).
These larvae after metamorphosis lose their tails and develop into
oozooids, known as "nurses," which are asexual, and are characterised (Fig.
60, B) by the possession of nine muscle-bands, by the stigmata being few in
number and confined to the posterior end of the branchial sac, by an
otocyst on the left side of the body, by a ventrally-placed complex stolon
or "rosette organ" near the heart, from which primary buds are produced by
constriction, and by a dorsal outgrowth ("the cadophore") near the
posterior end of the body. The buds (blastozooids) give rise eventually,
after {98}further division, to the sexual generation, which is
polymorphic—having three distinct forms, in two of which the reproductive
organs remain undeveloped.
[Illustration: FIG. 60.—Life-history of _Doliolum_. A, tailed larval stage;
B, "nurse" or oozooid, showing buds (blastozooids) migrating from the
ventral stolon to the dorsal process; C, posterior part of much later
oozooid to show buds arranged in three rows on dorsal process; D, stolon
segmenting; E, young migrating bud; F, trophozooid developed from one of
the buds of a lateral row. _At_, Atrial aperture; _b_, buds; _Br_,
branchial aperture; _cl_, cloaca; _d.p_, dorsal process; _end_, endostyle;
_ht_, heart; _l.b_, lateral buds; _m.b_, median buds; _n.g_,
nerve-ganglion; _ot_, otocyst; _p.c_, pericardium; _sk_, stalk; _sto_,
stolon. (After Uljanin and Barrois.)]
The primary buds are constricted off while still very young and undeveloped
(Fig. 60, D, B, and E); they migrate from their place of origin on the
stolon, over the surface (aided by large amoeboid test-cells which become
attached to the buds) (Fig. 60, B), multiply by fission, and become
attached (again by the help of amoeboid test-cells and ectoderm cells which
form a slight "placenta") in three rows—a median and two lateral—to the
dorsal outgrowth (Fig. 60, C) of the body of the nurse. This parent-form by
this time has become greatly modified, and its structure is largely
sacrificed for the good of the buds or growing zooids, for which it really
forms a locomotory organ. Its muscle-bands become greatly developed in
width (Fig. 60, C), and the branchial meshwork, endostyle, and alimentary
canal disappear.
The three forms produced in the second generation are as follows:—(1)
Nutritive forms ("trophozooids") derived from the lateral rows of buds,
which remain permanently attached to the {99}oozooid, and are sacrificed
for the benefit of the rest of the colony. They serve merely to aid in
respiration, and to provide the food for the nurse and the median buds.
Their development is arrested; they have the body elongated dorso-ventrally
with a large funnel-like branchial aperture (Fig. 60, F), and the
musculature is very slightly developed.
(2) Some of the median buds become foster forms ("phorozooids"), which,
like the preceding trophozooids, do not become sexually mature, but, unlike
them, are eventually set free as cask-shaped bodies having the _Doliolum_
appearance, with eight encircling muscle-bands, and having, moreover, a
ventral outgrowth (not a stolon), which is formed of the stalk by which the
body was formerly attached to the dorsal process of the oozooid. On this
ventral outgrowth the "gonozooids" (3) are attached while still very young
buds, and after the phorozooids are set free these reproductive forms
gradually attain their complete development, become sexually mature, and
are eventually separated off, finally losing all trace of their temporary
connexion with the foster-forms. They resemble the foster-forms in having a
cask-shaped body with eight muscle-bands, but differ in the absence of a
ventral process, and in having the sexual reproductive organs fully
developed.
OCCURRENCE.—The best-known member of the genus is _Doliolum tritonis_,
Herdman, which was captured in the tow-nets in thousands by Sir John Murray
during the cruise of H.M.S. "Triton" in the summer of 1882 in the North
Atlantic. Since then that species, or the closely allied _D. nationalis_,
Borgert, have been found on more than one occasion in the English Channel
and other parts of our south-west coast, and so _Doliolum_ may be regarded
as an occasional member of the British surface fauna.
It is probable that the occasional phenomenal swarms of _Doliolum_ which
have been met with in summer in the North Atlantic are a result of the
curious life-history which, under favourable circumstances, allows of a
small number of oozooids producing from minute buds an enormous number of
phorozooids and gonozooids.
As the result of the careful quantitative work of the German "Plankton"
expedition, Borgert thinks that the temperature of the water has more to do
with both the horizontal and the {100}vertical distribution of these
Thaliacea in the sea than any other factor.
OTHER GENERA.—_Anchinia_, of which only one species is known, _A. rubra_,
Vogt, from the Mediterranean, has the sexual forms permanently attached to
portions of the dorsal outgrowth from the body of the unknown oozooid
("nurse"). The stolon is probably much longer than in _Doliolum_, and
curves round so as to reach and lie along the dorsal outgrowth, upon which
it places the buds.
The body of the adult is elongated dorso-ventrally. The test is well
developed and contains branched cells. The musculature is not so well
developed as in _Doliolum_. There are two circular bands at the anterior
end, two at the posterior, and two muscles on the middle of the body, which
unite to form the characteristic S-shaped lateral bands. The stigmata are
confined to the obliquely-placed posterior end of the branchial sac. The
alimentary canal forms a U-shaped curve. The reproductive organs are placed
on the right side of the body. The life-history is still imperfectly known.
As in the case of _Doliolum_ the sexual generation is polymorphic, and has
three forms, two of which remain in a rudimentary condition so far as the
reproductive organs are concerned. They are known as the first and second
sterile forms, or "trophozooids." In _Anchinia_, however, the three forms
do not occur, so far as we know, together at the same time on the one
outgrowth, but are produced successively, or in different regions, the
reproductive forms of the sexual generation being independent of the
"foster-forms."[106]
The third genus, _Dolchinia_, contains also only a single species, _D.
mirabilis_, found by Korotneff[107] in the Gulf of Naples. It must have
three different forms in its life-history—oozooid, phorozooid, and
gonozooid, but the first of these is still unknown. On what must be body
processes detached from the oozooid are found phorozooids somewhat like
those of _Doliolum_, bearing sexual forms attached to ventral stalks.
_Dolchinia_ is intermediate on the whole between _Anchinia_, the most
simple member of the family, and _Doliolum_ the most complex; and may
eventually come to be united with the latter genus.
{101}SUB-ORDER 2. HEMIMYARIA.
Free-swimming pelagic forms which exhibit alternation of generations in
their life-history, and in the sexual condition form colonies. The body is
more or less fusiform, with the long axis antero-posterior, and the
branchial and atrial apertures nearly terminal and opposite. The test is
well developed but transparent. The musculature of the body-wall is in the
form of a series of transversely-running bands which do not usually form
complete independent rings as in the CYCLOMYARIA. These
partially-encircling muscles in the Salpidae (see Fig. 61, _m.b_) are
probably to be regarded as modified branchial and atrial sphincters which
have spread over the intervening body. The branchial and peribranchial
(cloacal) cavities form a continuous space in the interior of the body,
opening externally at the ends by the branchial and atrial apertures, and
traversed obliquely from the dorsal and anterior to the ventral and
posterior end by a long narrow vascular ciliated band, which represents the
dorsal lamina, the dorsal blood-sinus, and the neighbouring parts of the
dorsal edge of the branchial sac of an ordinary Ascidian. The alimentary
canal is placed ventrally. It may either be stretched out so as to extend
for some distance anteriorly, or, as is more usual, be concentrated to form
along with the testis a rounded opaque mass near the posterior end of the
body, known as the visceral mass or "nucleus." The embryonic development is
direct, no tailed larva being formed. The embryo is united to the parent
for a time by a "placenta."
This sub-order contains, in addition to its typical members, the SALPIDAE,
another still somewhat problematical family the OCTACNEMIDAE, including a
single very remarkable deep-water genus (_Octacnemus_), which in some
respects does not conform with the characters given above, and exhibits a
certain amount of affinity with the primitive fixed forms from which
Salpidae have been derived.
{102}[Illustration: FIG. 61.—_Salpa runcinata-fusiformis_. A, aggregated or
"chain" form; B, solitary form. _At_, Atrial aperture; _at.m_, atrial
muscles; _Br_, branchial aperture; _br.m_, branchial muscles; _d.l_, dorsal
lamina or "gill"; _d.t_, dorsal tubercle; _emb_, embryo; _end_, endostyle;
_m_, mantle; _m.b_, muscle-bands; _n.g_, nerve-ganglion; _p.p_,
peripharyngeal bands; _st_, stolon; _st″_, "chain" of buds; _t_, test; _v_,
visceral "nucleus."]
[Illustration: FIG. 62.—Diagram to show the arrangement and connexion of
the aggregated zooids in a young chain of Salps. 1, 3, 5, zooids on the
right; 2, 4, 6, zooids on the left. _At_, Atrial aperture of a zooid; _Br_,
branchial aperture of another; _c.t_ at the top of the figure points to
three pairs of connecting tubes; _c.t_ at the foot, to two pairs. Each
zooid is united to each of the four neighbours it touches by a pair of
connecting tubes, and so has eight such tubes in all.]
OCCURRENCE AND REPRODUCTION.—The family SALPIDAE[108] includes the single
genus _Salpa_, Forskål, which, however, may be divided into two well-marked
groups of species—(1) those such as _S_. (_Cyclosalpa_) _pinnata_, in which
the alimentary canal is stretched out ("ortho-enteric" condition) along the
ventral surface of the body, and (2) those such as _S.
runcinata-fusiformis_, in which the alimentary canal forms a compact
globular mass (Fig. 61, _v_), the "nucleus" ("caryo-enteric" condition),
near the posterior end of the body. About fifteen species altogether are
known; they are all pelagic in habit, and are found in nearly all seas.
Each species occurs in two forms (Fig. 61, A and B), the solitary asexual
(_proles solitaria_), and the aggregated sexual (_proles gregaria_), which
are in most species quite unlike one {103}another, the aggregated form
being usually more rounded, ovoid, or fusiform (Fig. 61, A), and the
solitary more quadrangular, and often provided with conical processes or
projecting points.
[Illustration: FIG. 63.—Diagram to show the relations of the groups of
young buds, when first formed on the stolon of _Salpa_. _at_, Atrial
aperture; _br_, branchial aperture; _el_, elaeoblast; _end_, endostyle;
_h_, heart; _n.g_, nerve-ganglion; _ov_, ovum; _s_, stolon; _st_, stomach;
I, II, III, groups of buds. (After Brooks.)]
[Illustration: FIG. 64.—Transverse section through endostyle and young
stolon of _Salpa pinnata_. _ec_, Ectoderm of parent reflected at _ec′_ to
cover base of stolon; _ec″_, ectoderm of stolon; _end_, endoderm of stolon;
_g_, ovary; _mes_, mesoderm cells; _n_, nerve-tube of stolon; _p.br_,
peribranchial tubes of stolon. (After Brooks.)]
The solitary form gives rise, by gemmation at the posterior end of the
endostyle (Fig. 63), to a complex tubular stolon, containing processes from
the more important organs of the parent-body, which give rise to an
endodermal tube, two peribranchial tubes, a neural tube, two blood-sinuses
and mesoblast cells, a genital cord, and over all the ectodermal covering
(see Fig. 64). This stolon becomes segmented (Fig. 63) into a series of
buds or young "chain" individuals, of which there may be several hundreds.
As the stolon elongates (Fig. 61, B, _st″_), the buds {104}undergo lateral
shifting, and rotation round their longitudinal axis, so as to acquire the
relations seen in the "chain," which then emerges from the tube in the test
through which it has been growing, so as to project to the exterior near
the atrial aperture. The buds at its free end which have now become far
advanced in their development are set free in groups, which remain attached
together by processes of the test, each enclosing a diverticulum from the
body-wall (Fig. 62), so as to form "chains." Each member of the chain is a
_Salpa_ of the sexual or aggregated form, and when mature may—either still
attached to its neighbours or separated from them—produce one or several
embryos (Fig. 61, A, _emb_), which develop into the solitary form of
_Salpa_. Thus the two forms, different in appearance and structure and
different in mode of origin, alternate regularly in the life-history of
_Salpa_.
STRUCTURE.—The more important points in the structure of a typical _Salpa_
are shown in Fig. 65. The branchial and atrial apertures are at opposite
ends of the body, and lead into large cavities, the branchial and
peribranchial sac respectively, which are in free communication at the
sides of the obliquely-running dorsal lamina or "gill" (_d.l_). The
transparent test is usually thick, and varies from a gelatinous to a stiff
cartilaginous condition; it adheres closely to the surface of the mantle
(ectoderm and body-wall). The muscle-bands (from 4 to about 20—usually 8 or
10) of the mantle do not in most cases completely encircle the body. They
are present dorsally (Fig. 65, _mus.bds_) and laterally, but the majority
do not reach the ventral surface. In many cases neighbouring bands join in
the median dorsal line (Fig. 61). The muscle fibres are striated, and have
rows of large equidistant nuclei. The anterior end of the dorsal lamina is
in some cases prolonged to form a prominent tentacular organ, the languet
or dorsal tentacle, projecting into the branchial sac, while near this
opens a ciliated funnel corresponding to the dorsal tubercle, but having no
connexion in the adult with either ganglion or subneural gland. The
conjoined ganglion and subneural gland, the dorsal lamina, the
peripharyngeal bands and the endostyle are placed in the usual positions.
Eyes in the form either of a continuous horse-shoe-shaped pigmented ridge
on the dorsal surface of the ganglion immediately below the ectoderm, or of
one larger median and several smaller lateral ocelli are found in the
various species of _Salpa_. These eyes have in {105}most cases a retina
formed of elongated cells, and a pigment-layer placed upon the ganglion.
The so-called otocysts of _Salpa_ have been shown by Metcalf to be really
glandular organs. They have been called lateral neural glands; they do not
open at the dorsal tubercle, but separately into the pharynx. These lateral
neural tubular glands have also been regarded as nephridia.
The large spaces at the sides of the dorsal lamina (often called the gill
or branchia of _Salpa_), by means of which the cavity of the branchial sac
is placed in free communication with the peribranchial cavity, are to be
regarded as gigantic gill-slits formed by the suppression of the lateral
walls and small stigmata of the branchial sac. The alimentary canal at the
posterior end of the "gill" consists of oesophagus, stomach, and intestine,
with a pair of lateral gastric glands or caeca. These viscera along with
the reproductive organs, when present, make up the "nucleus" (Fig. 66,
_v_).
[Illustration: FIG. 65.—Diagrammatic sagittal section of a "chain" _Salpa_.
_an_, Anus; _at_, atrial aperture; _at.m_, muscles of atrial aperture;
_atr.cav_, atrial cavity; _br_, branchial aperture; _br.m_, muscles of
branchial aperture; _br.s_, branchial sac; _d.l_, dorsal lamina or "gill";
_d.t_, dorsal tubercle; _end_, endostyle; _ht_, heart; _int_, intestine;
_l_, sensory languet; _mus.bds_, muscle-bands; _n.g_, nerve-ganglion; _oc_,
eye-spot; _oe_, oesophagus; _ov_, ovary; _p.p.b_, peripharyngeal band;
_s.gl_, neural gland; _stom_, stomach; _t, t′_, test; _tes_, testis; _z_,
prebranchial zone. (After Herdman.)]
ALTERNATION OF GENERATIONS.—Fig. 66 represents an aggregated or sexual
_Salpa_, which was once a member of a chain, since it shows a testis and a
developing embryo. The ova (always few in number, usually only one) appear
at a very early period in the developing chain _Salpa_, while it is still a
part of the gemmiparous stolon in the body of the solitary _Salpa_. This
gave rise to the view put forward first by Brooks that the ovary
{106}really belongs to the solitary stolon-bearing _Salpa_, which is
therefore a female producing a series of males by asexual gemmation, and
depositing in each of these an ovum, which will afterwards, when
fertilised, develop in the body of the male into a solitary or female
_Salpa_. This idea, if adopted, would profoundly modify our conception of
_Salpa_ as an example of a life-history showing alternation of generations,
but it seems to me to give a distorted view of the sequence of events. The
fact that the stolon while in the solitary _Salpa_ contains, along with
representatives of other important systems of the body, a row of germinal
cells, does not constitute that solitary _Salpa_ the parent of the ova
which these germinal cells will afterwards become in the body of an
independent bud. We must regard as the parent the body in which the ova
become mature and fulfil their function. The sexual or chain _Salpa_,
although really hermaphrodite in its life-history, is usually[109]
protogynous, _i.e._ the ova mature at an earlier period than the male organ
or testis. This prevents self-fertilisation. The ovum is presumably
fertilised by the spermatozoa of an older _Salpa_ belonging to another
chain, and the embryo is far advanced in its development before the testis
is formed. The development takes place inside the body of the parent, and
is "direct"—no tailed larval form being produced.
[Illustration: FIG. 66.—_Salpa hexagona_, Q. and G. Chain form dissected
from the left side. _a_, Anus; _at_, atrial aperture; _br_, branchial
aperture; _d.l_, dorsal lamina ("gill"); _d.t_, dorsal tubercle; _emb_,
embryos; _end_, endostyle; _m.b_ 2, _m.b_ 7, second and seventh
muscle-bands; _n.g_, nerve-ganglion; _v_, visceral "nucleus." (After
Traustedt.)]
DEVELOPMENT AND LIFE-HISTORY.—The segmentation of the egg is holoblastic,
and gives rise to a number of blastomeres, {107}which are for a time masked
by the phenomenal activity of certain cells of extraneous origin, the
"kalymmocytes," derived from the follicular epithelium surrounding the
ovum. These follicular kalymmocytes migrate into the ovum, surround groups
of blastomeres, and arrange themselves so as to reproduce the essential
structure of the future embryo for which they form what may be termed a
scaffolding or temporary support. After a time the blastomeres become
active, proliferate rapidly, and finally press upon and absorb the
kalymmocytes, and so eventually take their proper place in building up the
organs. Some observers regard the kalymmocytes as being passive and
nutritive only in function.
[Illustration: FIG. 67.—Young solitary _Salpa democratica-mucronata_
attached to the parent by the placenta. _atr.ap_, Atrial aperture; _br_,
dorsal lamina; _cil.gr_, dorsal tubercle; _ebl_, elaeoblast; _end_,
endostyle; _n.gn_, nerve-ganglion; _oes_, oesophagus; _or.ap_, branchial
aperture; _peric_, pericardium; _pl_, placenta; _rect_, intestine; _stol_,
stolon; _stom_, stomach. (From Parker and Haswell, after Salensky.)]
At an early period in the development a part of the surface of the embryo,
on its ventral edge, becomes separated off, along with a part of the wall
of the cavity ("oviduct"—a diverticulum from the atrium) in which it lies,
to form the "placenta" (Fig. 67, _pl_) in which the embryonic and maternal
blood-streams circulate in close proximity, and so allow of the conveyance
of nutriment to the developing embryo by means of large migrating placental
cells. At a somewhat later stage a number of cells placed at the posterior
end of the body alongside the future nucleus become filled up with
oil-globules to form a mass of nutrient material—the "elaeoblast" (Fig. 67,
_ebl_)—which is used up later in the development. Many suggestions have
been made as to the homology and meaning of the elaeoblast; but it may now
be regarded as most probable that it is reserve food-material associated
with the disappearing rudiment of the {108}tail found in the larval
condition of most Ascidians. The development is direct; and it may be said,
then, that this young asexual (solitary) _Salpa_ differs from the
corresponding form in the life-history of _Doliolum_ (Fig. 60, A) in that
its tail is no longer a locomotory organ, but is represented by a nutritive
mass, the elaeoblast, while the body, in place of being free, is attached
by its ventral surface to a special organ of nutrition—the "placenta"—in
connexion with the blood-stream of the parent.
This embryo sexually produced inside the body of an aggregated form becomes
a solitary _Salpa_ (such as Fig. 61, B), which differs in appearance,
structure, and habits from its parent, and has no reproductive organs.
After swimming for a time, however, it develops the ventral stolon on which
buds form which are eventually sexual Salpae. These are set free from the
solitary form in sets, still connected together, and they may swim about
together for a time as a chain of aggregated Salpae before separating to
become the adult sexual individuals (such as Fig. 61, A).
CLASSIFICATION.—_Salpa_ may be divided into the following
subgenera:[110]—_Cyclosalpa_, Blainville, in which the alimentary canal is
ortho-enteric, and the "chain" consists of individuals united in a circle;
_Iasis_, Savigny, with several embryos formed at a time; and _Pegea_, Sav.,
_Thalia_, Blumenbach, and _Salpa_, Forskål, all with one embryo only, and
differing from one another in the condition of the "gill" and other
details: all except _Cyclosalpa_ have the alimentary canal caryo-enteric.
_Cyclosalpa_ has three species, the best known of which is _C. pinnata_ of
the Mediterranean, a form possessing light-producing organs like those of
_Pyrosoma_, but placed along the sides of the body. _Salpa_ has four or
five species, one of which, _S. runcinata-fusiformis_ (Fig. 61), has
occasionally been found in British seas; _Thalia_ includes the species _T.
democratica-mucronata_, which has been sometimes obtained in swarms in the
Hebridean seas, or cast ashore on our southern or western coasts; _Pegea_
has the species _P. scutigera-confoederata_; and _Iasis_ contains the
remaining half-dozen species, the best known of which is _I.
cordiformis-zonaria_, the only other Salpian which has been found in
British seas.
{109}[Illustration: FIG. 68.—A, solitary form of _Octacnemus bythius_
(after Moseley); B, diagram of structure of _Octacnemus_ (after Herdman);
C, aggregated form of _O. patagoniensis_ (after Metcalf). 1, from outside;
2, with test removed; and 3, with mantle removed. _a_, Anus; _adh_, area of
attachment; _at_, atrial, and _br_, branchial aperture; _br.s_, branchial
sac; _end_, endostyle; _g.s_, gill-slits; _i_, intestine; _n.y_,
nerve-ganglion; _oe_, oesophagus; _ov_, ovary; _p.br_, peribranchial
cavity; _st_, stomach; _stol_, stolon.]
The family OCTACNEMIDAE includes the single remarkable genus _Octacnemus_,
now known in a solitary and an aggregated form. It was found during the
"Challenger" expedition, and was first described by Moseley. It is
apparently a deep-sea representative of the pelagic Salpidae, and may
possibly be fixed at the bottom. The body in the solitary form is somewhat
discoid, with its margin prolonged to form eight tapering processes, on to
which the muscle-bands of the mantle are continued. The alimentary canal
forms a compact nucleus, which is attached to an apparently imperforate
membrane which stretches across the body, separating the branchial from the
atrial cavities. The endostyle is very short, and the dorsal lamina is also
much reduced. The reproduction and life-history are entirely unknown. The
aggregated form consists of a small number of individuals united by a
slender cord composed of test, body-wall, and endodermal tissue.
_Octacnemus_ has been found[111] in the South Pacific from depths of 1070
and 2160 fathoms, and off the Patagonian coast from 1050 fathoms. Two
species have been described: _O. bythius_, Moseley, and _O. patagoniensis_,
Metcalf. Metcalf, who has recently investigated the aggregated form (_O.
patagoniensis_), considers that the genus is more nearly related to the
Clavelinidae than to the Salpidae. Possibly its position might be best
{110}indicated by a line diverging from near the point (3) in the
phylogenetic diagram below.
GENERAL CONCLUSIONS.
The following diagram is a graphic representation of the genetic
affinities, or what is now generally supposed to have been the probable
course of phylogeny of the Tunicata. It will be noticed that it shows (1)
the Proto-Tunicates arising from Proto-Chordata, not far from the ancestors
of Amphioxus (see also, this vol. p. 112); (2) that the Larvacea are
regarded as the most primitive section of the group; (3) that the Thaliacea
(Doliolidae and Salpidae) are supposed to be derived not directly from
primitive pelagic forms, but through the early fixed Ascidians, not far
from (4) the ancestral compound Ascidians, which gave rise to the
Pyrosomatidae; (5) that the Ascidiidae and other higher Simple Ascidians
are derived, like the Compound Ascidians, from ancestral Clavelinidae; and
(6), that the Ascidiae Compositae are polyphyletic, the Holosomata
(Botryllidae and Polystyelidae) being derived from ancestral Simple
Ascidians independently of the Merosomatous families.
↑
| Molgulidae
| Ascidiidae |
+––––––––Amphioxus (5) | |
P | +–––––––+–––––+–––––+––––––+––– Cynthiidae.
r | | | |
o | +– C | |
t | Larvacea. | l | (6) Polystyelidae
o | | | a |
c | (1) (2) | +– v Botryllidae
h +––––––––––––––+––––––––––––| e
o | Proto–Tunicata. | l
r | +– i Distomatidae
d | Doliolidae–+ | n |
a | | | i | Didemnidae and
t | | | d | | Diplosomatidae
a | | | a | |
| | (3)| e (4) | |
| +–––––––+–––––––+––––+––––––––––+––––Polyclinidae
| Salpidae–––+ |
| Pyrosomatidae
The Tunicata are remarkable for the variety in appearance, structure, and
life-history which they present. No group illustrates in a more instructive
manner so large a number of important biological principles and phenomena.
They show solitary and colonial forms, fixed and free, pelagic and abyssal.
The development is in some cases larval and with metamorphosis, in others
abbreviated and direct. Persistent traces of ancestral characters are seen
in the embryonic and larval stages, while the adults present the most
varied secondary adaptations to littoral, {111}pelagic, and deep-sea,
free-swimming and sessile modes of existence. In the details of their
classification they demonstrate both stable and variable species,
monophyletic and polyphyletic groups. They exhibit the phenomena of
gemmation and of embryonic fission, of polymorphism, hibernation,
alternation of generations, and change of function. They have long been
known as a stock example of degeneration; but in fact they lend themselves
admirably to the exposition of more than one "Chapter of Darwinism."
* * * * *
NOTE TO P. 78.—_Oligotrema_, Bourne (_Quart. J. Micr. Sci_. xlvii. Pt. ii.
1903, p. 233), a Molgulid from the Loyalty Islands, has a reduced branchial
sac and greatly developed pinnate, muscular branchial lobes, probably used
in capturing food.
{112}CHAPTER IV
CEPHALOCHORDATA
INTRODUCTION—GENERAL CHARACTERS—ANATOMY OF AMPHIOXUS—EMBRYOLOGY AND
LIFE-HISTORY—CLASSIFICATION OF CEPHALOCHORDATA—SPECIES AND DISTRIBUTION
The CEPHALOCHORDATA comprise only a small group of little fish-like forms,
the Lancelets, usually known as "Amphioxus," and referable to about a dozen
species arranged in several closely allied genera under the single family
Branchiostomatidae. The best known form is _Branchiostoma lanceolatum_
(Pallas), the common Amphioxus or Lancelet, which has been found in British
seas, and even as far north as the coast of Norway, but is much more common
in warmer waters, such as the Mediterranean, and is also found in the
Indian Ocean. It is abundant in the Bay of Naples, and lives and breeds in
great numbers in a salt lagoon, the "Pantano," near Messina, and from these
localities most of the specimens have been obtained for the numerous recent
researches upon its structure and development.
Amphioxus was first discovered and described (1778) by Pallas, who regarded
it as a Mollusc, and named it _Limax lanceolatus_. It was first correctly
diagnosed as a low Vertebrate, and named _Branchiostoma_, by Costa, in
1834. The term _Amphioxus_, under which it has become so well known, was
applied to it a couple of years later by Yarrell.
The anatomy was for the first time fully investigated by Johannes Müller in
1841, and this important memoir has been supplemented in regard to special
systems and histological details by numerous papers by many leading
zoologists, such as those by Huxley in 1874, Langerhans in 1876, Lankester
in {113}1875 and in 1889, Retzius in 1890, and Boveri and Hatschek, both in
1892. Important papers on special points have also been written by Rolph,
Rohde, Benham, Andrews, Goodrich, and others. The development was first
elucidated by Kowalevsky in 1867, at about the same time when he studied
the development of the Ascidians, and later again in 1877. Further papers
on the development and metamorphosis we owe to Hatschek in 1881, Lankester
and Willey in 1890 and 1891, Wilson in 1893, and quite recently to
MacBride. Dr. Willey's book, _Amphioxus and the Ancestry of the Vertebrata_
(1894), contains a summary of investigations on structure and development,
an interesting discussion of the relations of Amphioxus to the other
Chordata, and a full bibliography.
In addition to such original researches, Amphioxus is studied in more or
less detail every year by countless senior and junior students in
zoological laboratories and marine stations throughout the civilised world.
The value of this primitive form as an object of biological education
depends upon the fact that it shows the essential Vertebrate characters,
and their mode of formation, in a very simple and instructive condition.
Although no doubt somewhat modified, and possibly degenerate in some
details of structure, in its general morphology it presents us with a
persistent type probably not far removed from the ancestral line of early
Chordata. There are no sufficient grounds for the view that Amphioxus is a
very degenerate representative of fish-like Vertebrata.
GENERAL CHARACTERS.—The Cephalochordata (or Acrania, in contradistinction
to the Craniata or Vertebrata) are marine, non-colonial Chordata, in which
the notochord extends the entire length of the body, running forward into
the snout beyond the nervous system. There is no skull, and the notochord
is not surrounded by any vertebral column. There are no limbs nor paired
fins. There is no exoskeleton, and the ectoderm is a single layer of
non-ciliated columnar cells. The mouth is ventral and anterior, the anus is
ventral, posterior, and asymmetrically placed on the left side. The pharynx
is a large branchial sac, having its sides perforated by many gill-slits,
and is surrounded by an ectodermal enclosure, the atrium, which opens to
the exterior by a median ventral atriopore. The stomach gives off a simple
saccular pouch, the liver, which has {114}connected with it a simple
hepatic portal blood system. There is a respiratory circulation, the
contractile ventral vessel which represents the heart sending the
colourless blood forward to the respiratory pharynx to be purified. The
body-wall is segmented into over fifty myotomes. There are numerous
separate nephridia which develop from the mesoderm and open into the
atrium. The brain remains undeveloped, being scarcely distinct from the
spinal cord. There are two pairs of cerebral nerves, and many spinal, in
which the dorsal and ventral roots or nerves do not unite. The sense-organs
are simple; there are no paired eyes and no auditory organs. The sexes are
separate; the gonads are metamerically arranged on the body-wall, and have
no ducts: they burst into the atrium. In the development the segmentation
is complete, a gastrula is formed by invagination, the nervous system is
formed from the dorsal epiblast, the notochord from the hypoblast, and the
mesoderm arises from metameric coelomic pouches. The body-cavity is an
enterocoele. The gill-slits are at first perforations of the body-wall
opening from the pharynx to the exterior, which later become enclosed by
the development of the atrium.
ANATOMY.
EXTERNAL CHARACTERS.—Amphioxus[112] is about 1½ to 2½ inches in length,
slender, somewhat translucent, and pointed at both ends (Fig. 69). It lives
in shallow water and burrows in the sand, head first, with great rapidity.
It frequently remains with the anterior end protruding from the sand. When
on the surface it lies on one side. It is said to swim freely at night. The
head end is rather the thicker, and the anterior two-thirds of the ventral
surface are flattened (Fig. 70, A), and may be slightly ridged
longitudinally. The lateral edges of this flat area project as metapleural
folds (Fig. 70, _mt.pl_), which begin anteriorly at the edges of the
external mouth, and die away in the middle line posteriorly behind a median
opening, the atriopore (Fig. 70, _atrp_). From this point a ventral median
fin (_vent.f_) extends backwards around the pointed posterior end
{115}(caudal fin, _cd.f_), and then forwards along the upper surface
(dorsal fin, _dors.f_) to the anterior end of the body. These fins thus
constitute a continuous median fold around a great part of the animal (Fig.
70, B, and Fig. 71).
[Illustration: FIG. 69.—Amphioxus (_Branchiostoma lanceolatum_) in the
Pantano at Messina. (After Willey.)]
[Illustration: FIG. 70.—_Branchiostoma lanceolatum_. A, ventral; B, side
view of the entire animal. _an_, Anus; _atrp_, atriopore; _cd.f_, caudal
fin; _cir_, cirri; _dors.f_, dorsal fin; _dors.f.r_, dorsal fin-rays;
_gon_, gonads; _mtpl_, metapleure; _myom_, myomeres; _nch_, notochord;
_or.hd_, oral hood; _vent.f_, ventral fin; _vent.f.r_, ventral fin-rays.
(After Kirkaldy.)]
The surface is soft all over, there being no exoskeleton. The epidermis or
ectoderm is formed by a single layer of {116}epithelial cells (see Fig. 72,
p. 118), some of which bear sensory processes, while others have a striated
cuticular border. There is no general ciliation of the surface in the
adult.
[Illustration: FIG. 71.—Diagram of the anatomy of Amphioxus. A, anterior;
B, posterior part. _an_, Anus; _atr_, atrium; _atr′_, its posterior
prolongation; _atrp_, atriopore; _br_, brain; _br.cl_, branchial clefts;
_br.f_, brown funnel; _br.sep.1_, primary, _br.sep.2_, secondary branchial
lamella; _br.r.1_, primary, _br.r.2_, secondary branchial rod; _caud.f_,
caudal fin; _cent.c_, central canal; _cir_, cirri; _coel_, coelom;
_dors.f_, dorsal fin; _dors.f.r_, dorsal fin-ray; _en.coe_, cerebral
vesicle; _e.sp_, eye-spot; _gon_, gonad; _int_, intestine; _lr_, liver;
_mth_, mouth; _myom_, myotomes; _nch_, notochord; _nph_, nephridia;
_olf.p_, olfactory pit; _or.f.hd_, oral hood; _ph_, pharynx; _sk_, skeleton
of oral hood and cirri (dotted); _sp.cd_, spinal cord; _vent.f_, ventral
fin; _vent.f.r_, ventral fin-ray; _vl_, velum; _vl.t_, velar tentacles.
(From Parker and Haswell.)]
The true mouth is a small pore at the bottom of a large vestibule (the
stomodaeum), placed at the anterior end of the ventral surface (Figs. 70
and 71), and formed by the "oral hood," which may be a prolongation
forwards of the atrial or metapleural folds at each side. The edges of the
oral hood bear 12 to 20 pairs of cirri (Fig. 70, _cir_) or ciliated
tentacles (strengthened by skeletal rods), which form a sensory fringe
around the opening. The anus (Figs. 70 and 71, _an_), is asymmetrical,
being {117}placed on the left side of the ventral fin, some distance behind
the atriopore, and not far from the posterior end of the body. The short
region behind the anus and surrounded by the caudal fin may properly be
called "tail." The current of water for respiratory and nutritive purposes,
and which may carry the ova and spermatozoa to the exterior, usually passes
in at the mouth and out at the atriopore, as in the Tunicata. On occasions,
however, it is said to be reversed.
GENERAL STRUCTURE.—The general plan of organisation of the body (see Fig.
71) is that a longitudinal skeletal axis, the notochord (_nch_), separates
a dorsal nervous system (_sp.cd_) from a ventral reduced coelom (_coel_),
in which lie the alimentary canal (_int_), the gonads (_gon_), and other
organs. Thus a transverse section of the body (see Fig. 72) shows the
typical Chordate arrangement of parts, and is comparable with a transverse
section of a tadpole, a young fish, or a larval Ascidian. A peribranchial
(_atr_) or atrial cavity (which is morphologically a part of the external
world shut in) lies external to the coelom and body-wall around the pharynx
and the greater part of the alimentary canal, and opens to the exterior by
the atriopore. As in the Tunicata, the perforations (gill-slits) in the
wall of the pharynx (_br.cl_) open into the atrial cavity and so indirectly
to the exterior.
MUSCULATURE.—The thick body-wall is largely formed by muscular tissue
metamerically segmented into about 60 myotomes (Fig. 71, _myom_). These
muscle-masses, which (as is usual in Vertebrata) are thickest dorsally at
the sides of the notochord and spinal chord (Fig. 72, _m_), are so arranged
as to present the appearance in a lateral view of the body of a series of
shallow cones (<<) fitting into one another and with their apices directed
forwards. The muscle fibres are striated, and run longitudinally along the
body from the anterior to the posterior edge of each myotome, so as to be
attached at their ends to the two septa of connective tissue which form the
boundaries of the myotomes. These septa, the myocommas, are conspicuous
features in the external appearance of the body (Fig. 70, B). They are not
arranged so as to be opposite one another on the two sides, but the
myotomes on the right and left sides alternate, as can be seen in a
transverse section (Fig. 74, A, p. 121).
{118}[Illustration: FIG. 72.—_Branchiostoma lanceolatum_. Diagrammatic
transverse section of the pharyngeal region, passing on the right through a
primary, on the left through a secondary branchial lamella. _ao_, Dorsal
aorta; _c_, dermis; _ec_, endostylar portion of coelom; _f_, fascia, or
investing layer of myotome; _fh_, compartment containing fin-ray; _g_,
gonad; _gl_, glomerulus; _k_, branchial artery; _kd_, pharynx; _ld_,
combined atrial and coelomic wall (ligamentum denticulatum); _m_, myotome;
_mt_, transverse muscle; _n_, nephridium; _n.ch_, notochord; _of_,
metapleural lymph space; _p_, atrium; _sc_, coelom; _si_, ventral aorta;
_sk_, sheath of notochord and spinal cord (_sp.cd_); _uf_, spaces in
ventral wall. (From Korschelt and Heider, after Boveri and Hatschek.)]
It is by means of these lateral muscle-bundles that the rapid vibration or
alternate bending of the body from side to side in swimming or burrowing
can be performed. There are usually, on each side, 35 myotomes in front of
the atriopore, 14 between the atriopore and the anus, and 11 postanal,
making 60 in all: some species have only about 50 myotomes, and some as
many as 85. (See {119}Classification, p. 137, where a list of the species
with the number of myotomes in each is given.)
There are also transverse muscles (Fig. 72, _mt_) extending across the
ventral surface in the region of the body enclosed by the metapleural
folds, and serving to compress the atrial cavity, and so aid in the
expulsion of its contents.
Outside the muscular layer of the body-wall the thin integument is formed
of a dermal layer of soft connective tissue, covered by the epidermis, a
single layer of columnar cells, many of which, especially on the oral
cirri, have sensory bristles.
SKELETON.—The endoskeleton consists of the notochord and some tracts of
modified connective tissue which support various parts of the body.
The notochord of this animal is noteworthy amongst Chordata for extending
practically the entire length of the body, including the head, from snout
to tip of tail (Fig. 71). It lies in the median plane, but nearer the
dorsal than the ventral surface (Fig. 72), and has the myotomes at its
sides, the nervous system above and the alimentary canal below. It is
elliptical in section, and tapers to the two ends. The nuclei of the
original notochordal cells are displaced to the dorsal and ventral edges,
and the greater parts of the cells, in the adult, are occupied by large
vacuoles filled with a fluid secretion, so as to form by their distended
condition a stiff elastic structure. This state of the cells, and the
appearance it gives rise to (Fig. 73), seen best in young specimens, is
very characteristic of notochordal tissue. Around the notochord lies a
sheath of connective tissue which is continuous with the similar sheath
around the nervous system and with the septa between the myotomes.
[Illustration: FIG. 73.—Median sagittal section of notochord of an
Amphioxus of 32 mm.]
In addition to these skeletal layers of connective tissue there is a
cartilage-like tract in the oral hood. This is jointed, or made up of
separate rod-like pieces, one at the base of each cirrus, into which it
sends a prolongation (Fig. 71, _sk_). The dorsal and ventral fins are
supported by single and double rows {120}respectively of what have been
called "fin-rays." They are short rods of gelatinous connective tissue,
each enclosed in a lymph space. Finally, the bars constituting the walls of
the pharynx between the gill-slits contain slender skeletal rods which run
obliquely dorso-ventrally, and are of a stiff, gelatinous nature (see Fig.
75, p. 122). This skeletal connective tissue consists in all cases of a
fibrous deposit or matrix produced by the layer of epithelium (ectodermal,
endodermal, or mesodermal) which adjoins the tissue.
ALIMENTARY CANAL.—This has, as its most noteworthy feature, the Chordate
characteristic that the pharynx gives rise to the respiratory organ (see
Figs. 71 and 74, A); and in size and prominence, both in side view and in
sections, the modified pharynx of Amphioxus is fairly comparable with the
branchial sac (pharynx) of many Tunicata (see Fig. 23, p. 51), and might be
called by the same name.
The small primitive mouth, at the bottom of the cavity bounded by the oral
hood (stomodaeum), has a membranous border, the velum (Fig. 71, _vl_), the
edges of which are prolonged into a circle of 10 or 12 (up to 16 in some
species) simple oral tentacles turned inwards towards the pharynx (compare
tentacles of Ascidians, p. 45).
The pharynx, by far the largest part of the alimentary canal, and extending
nearly half-way along the body, is more important as a respiratory than as
a nutritive organ. Its walls over nearly the whole extent are perforated by
a large, and indefinite, number (100 or more on each side) of gill-slits
which run on the whole dorso-ventrally, but in the contracted condition
seen in preserved specimens have their lower ends directed obliquely
backwards, so that a vertical transverse section may cut through a number
of such slits and the intervening branchial bars (Fig. 74, A, _kb_). These
bars, and therefore the slits between them, are of two orders, primary and
secondary, the latter being developed later in larval life as downgrowths
or "tongue-bars," one from the top of each primary gill-slit, so as to
divide it into two secondaries. The primary and the secondary (or tongue-)
bars can be distinguished from one another by their structure in the adult
animal (Fig. 75, A and B).
{121}[Illustration: FIG. 74.—_Branchiostoma lanceolatum_. A, transverse
section of the pharyngeal region. _a_, Dorsal aorta; _b_, atrium; _c_,
notochord; _co_, coelom; _e_, endostyle; _g_, gonad (ovary); _kb_,
branchial septa; _kd_, pharynx; _l_, liver; _my_, myotome; _n_, nephridium;
_r_, spinal cord; _sn_, _sn_, dorsal and ventral spinal nerves. B,
Transverse section of the intestinal region. _atr_, Atrium; _coel_, coelom;
_d.ao_, dorsal aorta; _int_, intestine; _myom_, myotome; _nch_, notochord;
_neu_, spinal cord; _s.int.v_, sub-intestinal vein. (From Parker and
Haswell's _Zoology_. A, From Hertwig, after Lankester and Boveri; B, partly
after Rolph.)]
It must be remembered that these branchial bars, or septa between the
gill-slits, are not merely portions of the wall of the pharynx, but are in
a sense portions of the body-wall as well, and correspond in nature, though
not in number, to the visceral arches in a Vertebrate lying between the
visceral clefts which open on the exterior. In the adult Amphioxus the
clefts in the wall of the pharynx do not open directly to the exterior, but
into the peribranchial cavity or atrium, which, however, is only formed at
a late larval period as an invagination or enclosure of ectoderm. Previous
to that the first formed gill-slits opened to the exterior in Amphioxus
(see larva, Fig. 86, p. 134), just as they do in a fish or a young tadpole.
The atrial cavity is therefore, from its origin, lined by ectoderm, and the
outer surface of a branchial bar is virtually a part of the outer surface
of the body. It is only natural then to find that each bar contains a small
section of the coelom in its interior, communicating dorsally and ventrally
with other parts of that cavity (see Figs. 75 and 76). There are also
blood-vessels which run in the branchial bars and their junctions. The
greater part of the epithelium covering a branchial bar is pharyngeal
epithelium or endoderm {122}(Fig. 75, _br.ep_), but the external, wider,
non-ciliated cells (Fig. 75, _at.ep_) are ectodermal cells lining the
atrium. The gelatinous skeletal rods in the primary bars are forked
ventrally, while those in the secondary bars are simple; and there are
other points of detail in which the two kinds of bar differ. These bars are
obviously more numerous in the adult than the myotomes, but in the young
larva the first formed gill-clefts are metamerically arranged, and then
later they increase greatly in number. It is the cilia covering the
pharyngeal epithelium on the branchial bars, possibly aided by the ciliated
tracts of the oral hood, which cause the current of water already alluded
to.
[Illustration: FIG. 75.—Transverse sections through primary (A) and
secondary (B) branchial bars of Amphioxus. _at.ep_, Atrial epithelium;
_bl.s_, blood spaces or "vessels"; _br.ep_, branchial epithelium; _coel_,
coelomic cavity in primary bar; _sk_, skeletal rods. (From Willey, after
Benham.)]
Transverse branchial junctions (synapticula) run across the branchial bars,
connecting them at frequent intervals, and these transverse connexions,
like the branchial bars, are supported by skeletal rods. Along the ventral
median line of the pharynx runs a groove, the endostyle or hypopharyngeal
groove, comparable with the similar structure in the branchial sac of
Tunicata. This longitudinal groove (Fig. 76, _gl_) is lined by ciliated
epithelium containing four tracts of gland cells (compare endostyle in
Ascidians, Fig. 20, p. 46). There is reason to believe that this organ is
the homologue of the thyroid gland of Vertebrata. As in the case of
Tunicata the endostyle secretes mucus, which is carried forwards by the
cilia to constitute a train with entangled food particles which pass back
dorsally to the stomach. At the anterior end the ciliated lips of the
endostyle diverge to {123}the right and left to encircle the front of the
pharynx as the peripharyngeal bands. These unite again dorsally to form the
epipharyngeal (or hyperpharyngeal) groove which leads backwards,
corresponding to the hypopharyngeal groove below (see Fig. 74, A), till the
posterior end of the pharynx is reached.
[Illustration: FIG. 76.—Transverse section of the ventral part of the
pharynx of Amphioxus. _c_, Coelom; _e_, endostyle; _gl_, endostylar glands;
_m.b.a_, median branchial artery; _p.b_, primary bar; _sk_, endostylar and
branchial rods and skeletal plates; _t.b_, tongue-bar. (After Lankester.)]
The remainder of the simple alimentary canal is straight, and is scarcely
differentiated into regions. A slight narrowing of the tube behind the
pharynx has been called the oesophagus, and a slight enlargement which
follows, the stomach. From this point the intestine tapers backwards to the
anus (Fig. 71, p. 116). The ventral edge of the stomach gives off a blind
pouch, the hepatic caecum or saccular liver, which runs forwards on the
right-hand side of the pharynx (Fig. 74, A, _l_). This is a digestive
gland, is lined with glandular epithelium, and apparently corresponds with
the liver of Vertebrata. There are no other digestive glands in connexion
with the alimentary canal of Amphioxus.
COELOM.—In the young larva there are at first (as in Balanoglossus) five
coelomic spaces, a median anterior "head-cavity," a pair of antero-lateral
"collar-cavities," and a pair of more posterior long lateral grooves from
which arise, in the later larva, the segmented myotomes and ventrally a
large coelomic space surrounding the alimentary canal and separating it
from the body-wall. In the adult animal, however, the coelom has been so
much displaced by the formation of the spacious atrium that in front of the
atriopore it can only be recognised as a series of canals and crevices. The
relations of coelom to atrium in the region of the intestine are {124}seen
in Fig. 74, B, and in the region of the pharynx in Fig. 74, A. Fig. 72
shows the distribution of the spaces more in detail (see also Fig. 71).
Beginning anteriorly, along the dorsal surface of the pharynx and beneath
the notochord run a pair of dorsal coelomic canals, one at each side of the
epipharyngeal groove; these give off ventral diverticula which pass down
the primary branchial bars of the pharyngeal wall and unite ventrally in a
median tube, the endostylar coelom (see Fig. 72, _ec_). At the posterior
end of the pharynx these dorsal and ventral canals unite in a narrow
coelomic space encircling the stomach, inside the wall of the atrium, and
sending an extension forwards around the liver (Fig. 74, A, _l_). In the
region of the intestine, behind the atriopore, the coelom is allowed to
expand to its primitive condition on the left-hand side (Fig. 74, B), but
is still reduced on the right side, where there is a prolongation of the
atrial cavity reaching nearly to the anus. All these coelomic spaces are
lined by a coelomic epithelium.
The BLOOD SYSTEM of Amphioxus, although as simple as that of a Chaetopod
worm, is undoubtedly laid down on the Vertebrate plan—even though there is
no distinct heart and the vessels are few and of simple structure.
Capillary networks are formed in some places, but the colourless blood also
extends into many lacunae or lymph spaces, such as those around the
fin-rays and in the metapleura. As in a typical lower Vertebrate, there is
a contractile ventral vessel (the ventral or branchial aorta, Fig. 77,
_v.ao_) running forwards under the alimentary canal to the pharynx, and
giving off on each side afferent branchial vessels, which pass up the
primary branchial bars and give off branches joining the vessels in the
secondary bars. These latter do not communicate directly with the ventral
aorta, but the vessels in all the branchial bars open dorsally by efferent
branchial vessels into the paired dorsal aortae (Fig. 77, _d.ao_), which
run backwards along the top of the pharynx, one at each side of the
epipharyngeal groove. In the vessels of the branchial bars and their
connectives the blood is aerated by the current of water passing through
the gill-slits, and so reaches the dorsal aortae in a purified condition.
The right-hand dorsal aorta is continued forward further into the snout
than its fellow of the other side, and is dilated at its extremity (Fig.
77). At the posterior end of the pharynx the paired dorsal aortae unite to
form the median dorsal aorta {125}which runs backwards, lying between
notochord and alimentary canal. This vessel gives off branches to the wall
of the intestine, and these break up into capillary networks (Fig. 77,
_cp_), from which the blood is collected by the median sub-intestinal vein.
This then flows forwards to pass by the hepatic portal vein to the ventral
edge of the saccular liver, in the wall of which it is distributed in a
capillary network. The blood is collected on the dorsal edge of the liver
by the hepatic vein, which runs posteriorly and then turns downwards and
forwards to become continuous with the posterior end of the ventral aorta
or "heart."
[Illustration: FIG. 77.—Diagram of the vascular system of Amphioxus.
_af.br.a_, Afferent branchial arteries; _af.br.a′_, similar vessels of the
secondary (tongue) bars; _br.cl_, gill-slits; _cp_, intestinal capillaries;
_d.ao_, paired dorsal aortae; _d.ao′_, median dorsal aorta; _ef.br.a_,
efferent branchial arteries; _hep.port.v_, hepatic portal vein; _hep.v_,
hepatic vein; _int_, intestine; _lr_, liver; _ph_, pharynx; _s.int.v_,
sub-intestinal vein; _v.ao_, ventral aorta. (From Parker and Haswell.)]
It is clear that this course of the circulation agrees with that of a
typical lower Vertebrate in all essential points:—(1) in having the main
artery a dorsal aorta in which the blood flows backwards; (2) in having a
ventral vessel representing the heart, and sending impure blood forwards to
the respiratory region of the alimentary canal to be aerated; and (3) in
having a hepatic portal system consisting of the capillaries of the liver,
through which the blood from the intestinal wall has to pass before
reaching the ventral vessel (heart).
RENAL EXCRETORY functions have been attributed to various organs in
Amphioxus, and it is quite possible that, in addition to the true nephridia
which are now known, other tracts of tissue in the body may be able to
eliminate nitrogenous waste matters. Such are certain clumps of columnar
epithelial cells on the floor of the atrium, and the single pair of large
brown atrio-coelomic funnels lying on the dorsal edge of the posterior end
of the {126}pharynx (Fig. 71, _br.f_). There are, however, a large number
(about 100 pairs) of minute nephridia, discovered (1890) by Weiss and by
Boveri independently, lying at the sides of the dorsal coelomic canals
above the pharynx, which must be regarded as the chief functional renal
organs. These are bent tubules, partly glandular and partly ciliated, each
giving off several caecal knobs (at first supposed to be open nephrostomes,
one shown at each end of the tubule and three along its upper surface in
Fig. 78), which project into the coelom, and opening by one nephridiopore
(on the lower surface, and opposite a tongue bar of the pharynx) into the
atrial cavity. The knobs, or closed nephrostomes, are surrounded by
peculiar, slender, club-shaped tubular and flagellated cells—which
Goodrich[113] has shown to correspond to the "solenocytes" in the nephridia
of Polychaete worms (see Fig. 79).
[Illustration: FIG. 78.—_Branchiostoma lanceolatum_. A nephridium of the
left side with part of the wall of the pharynx, as seen alive, highly
magnified. (From Willey, after Boveri.)]
{127}The CENTRAL NERVOUS SYSTEM is dorsal and tubular as in Vertebrates,
and lies in a connective-tissue sheath immediately above the notochord
(Figs. 71, etc., and 80, A). Posteriorly it tapers to a fine point a little
in front of the end of the notochord, but anteriorly it ends abruptly some
distance behind the anterior extremity of the notochord. The central canal
is connected with the dorsal surface by a median longitudinal cleft (Fig.
80, C), and at the anterior end it dilates to form the cerebral vesicle
(_c.v_) with which two simple sense-organs, an eye-spot (_e_) and an
olfactory pit (_olf_), are connected. A patch of ciliated epithelium in the
floor of the vesicle has been described as an "infundibular-organ." There
is also a surface dilatation of the dorsal cleft behind the cerebral
vesicle (_dil_). The nervous system as far back as this point may be
regarded as the brain, though scarcely distinguishable externally (Figs. 71
and 80, A) from the spinal chord behind. From this "brain" arise two pairs
of "cranial" nerves, the first (I.) from the anterior end, and the second
(II.) from the dorsal surface of the cerebral vesicle; both are in front of
the first myotomes of the body, and supply the pre-oral snout with nerves.
[Illustration: FIG. 79.—Nephridia. A, portion of a nephridium of
_Phyllodoce_, a marine Polychaete, for comparison with B, portion of a
nephridium of Amphioxus. These figures show the solenocytes with their
flagella projecting through the long tubes into the lumen of the excretory
organ, and demonstrate the essential similarity of the nephridia of
Amphioxus with those of Polychaete worms (after Goodrich).]
The spinal cord gives off a large number of spinal nerves segmentally
arranged, but, like the myotomes, not opposite and symmetrical on the two
sides, but placed alternately (Fig. 81). Moreover, the spinal nerves arise
on each side at two levels, there being a more dorsal series each arising
by a single root and {128}supplying the integument as well as the
transverse muscles, so as to be sensory as well as motor, and a ventral
series arising each by a number of roots (Fig. 81) and wholly motor in
function, as they supply only the myotomes. These two series may be
compared to the dorsal and ventral roots which in the Vertebrata join to
form a mixed spinal nerve.
[Illustration: FIG. 80.—_Branchiostoma lanceolatum_. A, brain and cerebral
nerves of a young specimen; B, transverse section through neuropore; C,
behind cerebral vesicle; D through dorsal dilatation. _ch_, Notochord;
_cv_, cerebral vesicle; _dil_, dorsal dilatation; _e_, eye-spot; _np_,
neuropore; _olf_, olfactory pit; _I_ and _II_, cranial nerves. (From
Willey, after Hatschek.)]
In addition to ordinary small nerve cells the central nervous system
contains certain large nerve cells with very long processes, the "giant
fibres," which extend through the greater part of the length of the spinal
cord. No trace of a sympathetic nervous system has been found.