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A CARTILAGINOUS FIXED SEA-SQUIRT, Pkcillusia (liat. size).

jerkings of the tail; and it thereupon starts on a free-swimming existence.
Before long the cellular structure of the notochord in the tail begins to disappear
by the formation of vacuities ; and eventually the whole structure becomes filled
with gelatinous matter. After a brief free existence it fixes itself by its muzzle to
some submarine object, with the tail stretched out ami generally motionless. In
a short time this appendage commences to shorten, and finally disappears, by being
drawn up into the body of the developing sea-squirt and absorbed. A further
process of development results in the production of the perfect sea-squirt; but it



SEA-SQUIRTS. 565

would be quite foreign to the scope of this work to enter into the details of the
metamorphosis; and we may conclude this portion of our subject by stating that
ascidians are probably the degenerate descendants of permanently free-swimming
forms provided with a complete notochord and nerve-tube ; both of which structures
are now in most cases only temporarily retained in the tails of the larvae.

Typical According to the classification adopted by Professor Herdmaii,

Ascidians. the tunicates may be divided into three orders, the first of which is
known as the Ascidiacea. This group includes both fixed and pelagic, simple arid
compound types, none of which are provided in the adult state with a tail and
retain no trace of a notochord ; the free-swimming forms constituting colonies, and
the simple types being generally fixed. The outer tunic is permanent and well
developed, generally increasing with the age of the individual ; and the muscular
structure of the inner tunic takes the form of an irregular network, and never of
hoop-like bands. The walls of the large branchial chamber are perforated by
numerous slits, opening into a single atrial cavity, which in turn communicates
with the exterior by means of the exhalent aperture of the tunic ; the vent opening
into the atrial cavity. Many of the forms reproduce their kind by budding, and in
most the sexually produced embryo develops into a tailed larva. The order is
divided into three subordinal groups, of which the first Ascidiae Simplices
includes fixed (rarely unattached, but never free-swimming) and generally solitary
forms, which very rarely reproduce by budding. When colonies occur, each of
their individual members has a distinct outer tunic of its own, the whole society
never being buried in a common investing mass. Four families are contained in
this suborder, each represented by a large number of genera. Omitting mention
of the first family, we may take as an example of the second (Cynthiidcc) the
genus Microcosmus, of which specimens are shown in the figures on pp. 561, 562.
As a family, these sea-squirts are characterised by being usually attached, and
sometimes stalked, although rarely free. The outer tunic is generally membranous
or leathery, but occasionally cartilaginous or covered with sand ; while the inhalent
aperture is usually, and the exhalent aperture invariably, provided with four lobes,
meeting together at the centre. The branchial chamber is longitudinally folded,
with its gill-slits straight ; and the tentacles may be either single or compound.
In the figured genus the body is attached and sessile, and the tunic, which is not
incrusted with a continuous coat of sand, is thin, leathery, and tough ; both its
apertures having four lobes, and the tentacles being compound. As an example of
the family Ascidiidce we may take the well-known Phallusia mammillata, from
the seas of North-Western Europe and the Mediterranean, which is shown in the
accompanying illustration, and is the sole representative of its genus. In the
family to which it belongs, the body is attached and usually sessile, although
rarely stalked; the inhalent aperture generally has eight, and the exhalent six
lobes"; and, as a rule, the outer tunic is either gelatinous or cartilaginous, although
it may be horny. The branchial chamber is devoid of folds, with the gill-slits
either straight or curved ; and the tentacles are simple and thread-like. In the
figured genus the body is erect and attached, and the outer tunic of a cartilaginous
nature ; its surface being mammillated in a very characteristic manner. It may
be mentioned here that all the simple sea-squirts of this group, when touched, emit



566



SEMIVERTEBRA TES.



a jet of water; and that some of them, like the one figured on p. 561, are used as
articles of food. To the same subfamily as Pfiallusia also belongs the extensive
genus Ascidia, in which the outer tunic is soft and flexible, instead of being
cartilaginous. A totally distinct subfamily is, however, indicated by the remark-
able deep-sea genus Hypobytkius, of which the two known species were obtained
at depths varying from six hundred to two thousand nine hundred fathoms,
during the voyage of the Challenger. Here we find the cup-shaped or pear-like





PEAR-SHAPED ASCIDIAN, Htjpobythius (fo uat. size).

body attached by a longer or shorter stem ; while the apertures are circular and
not closed by lobes. The outer tunic is cartilaginous, but soft and thin, although
thickened in places to form plates. The internal longitudinal bars usually found
in the branchial chamber are wanting in this genus; the gill-slits are small and
irregularly placed ; and the viscera form a compact irregular mass on the dorsal
side of this chamber. In (lie species here figured (II. <-nl i/code*}, which is from
the North Pacific, the stem is of great length, and the outer tunic thickened so
as to form a number of nodules or plates ; but in the South Atlantic form (H.






SEA-SQUIRTS. 567

nwseleyi) the stem is much shorter, and there is only a single plate, situated on
the dorsal side. Of very large dimensions, these deep-sea ascidians are decidedly
the most beautiful members of the class, and present some resemblance to the
glass-sponges. A totally different type of structure is presented by the last family
(Clavelinidce) of the suborder, in which the body of each individual is attached
by its posterior end, and usually by means of a stalk, to a creeping basal stolon,
or common mass, from which young individuals are produced by budding. The
outer tunic, which is usually thin and transparent, is in most cases gelatinous,
although occasionally cartilaginous ; and its circular apertures are but seldom
distinctly lobed. Folds are wanting in the branchial chamber, but longitudinal
bars may be present, although these lack the papillae found in the preceding
family ; and the gill-slits are straight. The tentacles resemble those of the last
family in their simple, thread-like form ; but the digestive tract is usually
extended behind the branchial chamber to form an abdomen. In addition to
the ordinary sexual reproduction, colonies may be formed by budding from the
common stolon. Ten genera are included in the family, from among which the
typical Clavelina is selected for illustration. Here the body is elongated and
club-shaped, but with no peduncle beyond the abdomen, and is attached to a
delicate, branched, creeping stolon, from which arise
the buds. The thin outer tunic is gelatinous or
cartilaginous, w T ith its circular apertures devoid of
lobes. The inner tunic is likewise thin, with its
muscles mainly longitudinal ; and the intestinal tract
is extended to form a well-marked abdomen. In its
restricted sense, the genus includes only half a dozen
small species from North -Western Europe and the
Mediterranean ; the one here figured (C. lepadiformis)
being characterised by the yellow or brown lines on
the region known as the thorax.

The second suborder of the typical sea-squirts-

Ascidise Composite includes fixed forms which repro- ( nat . size).

duce by buds so as to constitute colonies in which the
individuals are buried in a common investing mass, and thus possess no separate
tunics. The group includes seven families ; and Professor Herdman remarks that
as many of these have originated independently from simple forms, the whole
assemblage is to a certain extent an artificial one. In the first family (Botryllidce)
the colonies, as shown in the illustration on p. 568, usually form thin incrusta-
tions on seaweeds or stones, although they occasionally take the shape of thick
fleshy masses ; the individuals being arranged so as to form either circles or ellipses,
or in branching lines. The common apertures of discharge are distinct, and usually
furnished with lobes; the individual units are short, and show no division of
the body into regions; and the outer tunic, which is usually soft, is traversed
by numerous vessels with large terminal knobs. Internal longitudinal bars are
present in the large and well-developed branchial chamber, in which the gill-slits
are numerous : and the simple tentacles do not exceed sixteen in number. Budding
may take place either from the sides of the units constituting the colony or from




5 68



SEMIVER TEBRA TES.



the vessels in the tunic. Among the five genera constituting the family, the
typical Botryllus (figured on p. 572) has the colony thin and incrusting, with the

individuals arranged in a circular manner, whereas in
Hot rylloides they form ellipses or branching lines. In the
figured species of the latter (B. albicants), from North-
Western Europe and the Mediterranean, the colony is
pure white in colour, but in some it is purple with yellow
or green markings, and in others yellowish green. As
an example of forms in which the colony is thick and
massive, we may cite the genus Polycyclus. Passing over
the second family of the suborder, we come to the third
(Polyclinidcv), in which the colony is usually massive,
being sometimes incrusting, but in other cases lobed; or
even stalked. The arrangement of the individuals is
highly variable ; and the common apertures of discharge
are usually inconspicuous. Although of an elongated
form, the individuals usually differ from those of the
family last noticed by being divided into three regions ;
the inhalent aperture having six or eight lobes, while tin-
exhalent is frequently provided with a tongue-like process.
The gelatinous or cartilaginous outer tunic is frequently
stiffened by embedded grains of sand ; and the branchial
chamber is usually small and poorly developed, with
minute gill-slits and no internal longitudinal bars. The
tentacles are small and not numerous ; the digestive tract
is extended posteriorly to a considerable distance beyond
the extremity of the branchial chamber ; and budding takes

place from the end of the postabdominal region. The family is represented by well
nigh a score of genera, among which Amarucium may be selected as an example,
on account of its numerous species. Here the colony is massive, being sometimes

lobed or stalked ; the mode of arrange-
ment is usually compound and 'irregular ;
and the individuals are elongated, with
six lobes to the inhalent orifice, and the
postabdominal region elongated. The
species here figured (A. densum) is from
North- Western Europe, and is character-
ised by its greyish yellow colour, and
the abundance of sand in the tunic ; but

other kinds may be black, orange, or
A COMPOUND ASCIDIAN (Amarucium) IN WINTER ..."

CONDITION (nat. size). ros y red > or White. In our figure, a ShOWS

fully active individuals; while those in

the outer ring indicated by b assume a kind of torpid condition during the winter,
but give rise to fresh buds in the spring.

Omitting mention of the remaining families of the group just considered, we
come to the third and last suborder of the typical asc-idians, namely, the phos-




AN INCRUSTINO ASCIDIAN

(Botrylloides) ON A SEA-
WEED (nat. size).







SEA-SQUIRTS. 569

phorescent ascidians, A. Luciee. These are represented solely by the genus
Pyrosoma, which is thus the only member of the family Pyrosomatidce. These
ascidians are free-swimming pelagic forms, reproducing by buds in such a manner
as to form colonies in the shape of a sac ; such colonies sometimes attaining huge
dimensions. In the sack thus formed the constituent individuals are embedded in
such a manner that all their inhalent apertures open on its outer surface, while
their exhalent orifices are situated within the cylinder; the mouth of the sac
forming the common discharging aperture. The apertures of the units are not
lobed ; and the outer tunic is gelatinous and transparent, containing no hard
spicules, but provided with numerous minute cells. The branchial chamber is
well-developed, and the tentacles are simple. The first four individuals of the
colony grow in the form of buds from a rudimentary sexually-developed larva ;
the subsequent increase taking place by budding from a ventral posterior stolon.
The genus is represented only by four species, in one of which (P. elegans) the
individuals form regular oblique rows in the walls of the sac, while in the other
three they are arranged irregularly. The largest of all is P. spinosum, from the
Atlantic, in which the total length of the colony may be upwards of four feet ; this
species being distinguished by the surface of the sac being provided only with
short sharp spines, instead of with large processes of the tunic. It is to these
ascidians that the most beautiful phosphorescence of tropical seas is due, each colony,
when stimulated by a touch or shake of the water, giving forth a brilliant ball of
bluish light, which lasts for several seconds, as the organism floats along beneath
the surface, and then suddenly disappears. A colony is figured on p. 576.

Describing the luminosity produced by these ascidians, Bennett states that on
one occasion in the Australian seas, when he reached the deck, he observed a " broad
and extensive sheet of phosphorescence, extending in a direction from east to west,
as far as the eye could reach. The luminosity was confined to the range of animals
in this shoal, for there was no similar light in any other direction. I immediately
cast the towing-net over the stern of the ship, as we approached nearer the
luminous streak, to ascertain the cause of this extraordinary and so limited
phenomenon. The ship soon cleaved through the brilliant mass, from which,
by the disturbance, strong flashes of light were emitted ; and the shoal, judging
from the time the vessel took in passing through the mass, may have been a mile
in breadth. The passage of the vessel through them increased the light around to
a far stronger degree, illuminating the ship. On taking in the towing-net, it was
found half filled with Pyrosoma, which shone with a pale, greenish light; and
there were also a few shell-fish in the net at the same time. After the mass had
been passed through, the light was still seen astern, until it became invisible in the
distance." Frequently the phosphorescence is intermittent, periods of luminosity
alternating with intervals of darkness. Moseley writes that during the voyage
of the Challenger, " a giant Pyrosoma was caught by us in the deep-sea trawl.
It was like a great sac, with its walls of jelly about an inch in thickness. It was
4 feet in length, and 10 inches in diameter. When a Pyrosoma is stimulated by
having its surface touched, the phosphorescent light breaks out at first at the
point stimulated, and then spreads over the surface of the colony as the stimulus
is transmitted to the surrounding animals. I wrote my name with my finger on



570



SEMIVERTEBRA TES.



the surface of the giant Pyrosoina, as it lay on deck in a tub at night, and my
name came out in a few seconds in letters of fire."

Non-Luminous With the exception of the family just mentioned, and also of a

Pelagic Ascidians. seconc [ one w hich constitutes the third order, the present ordinal
group termed the Thaliacea includes the whole of the free-swimming pelagic
representatives of the class. Either simple or compound in structure, these
ascidians lack both a tail and a notochord in the adult, but have a persistent
outer tunic, which may be either feebly or fully developed. In the inner tunic
the muscles are arranged in the form of more or less nearly complete circular
bands, the contraction of which forms the motive agency of the creatures. The
branchial chamber has either two large openings, or a number of smaller gill-slits,
leading to a single atrial cavity ; the latter communicating with the exterior by
the exhalent aperture, and the vent opening within it. In all the members of the
group an alternation of generations takes place ; and this may be further com-
plicated by the individuals of a single generation being unlike one another.
During one period of existence temporary colonies may be formed, but these never
increase by the budding of the constituent units, which eventually separate from
one another and disperse.



f\




AN INDIVIDUAL OF A CHAIN-SALPA.

a, inlialeiit, and b, exhalent, orifice ; d, gill ; c, e, viscera ; /, eye(?) ; y, pedicle of union (nat. size).

The well-known salpse form a suborder Hemimyaria characterised by the
formation of temporary colonies in the sexual generation, and represent a family
(Salpidce) distinguished by the muscular bands of the inner tunic being incomplete
on the lower surface of the body. Pelagic in habit, and transparent in structure,
salpse have been not inaptly compared to a barrel with both ends knocked out ;
and really consist of little more than a huge pharynx, swimming through the
water, and taking in large mouthfuls of the same at each contraction of its
muscles. Through the hollow, to below the hinder aperture, runs obliquely a
rod-like gill (d) from above the mouth, although this is too narrow to interfere
with the free flow of the water; while the lower surface of the interior of the
creature is furnished with a ciliated slime-secreting band, corresponding to the
structure known in other ascidians and the lancelet as the endostylo. It may hero
be well to mention that in the lancelet the structure in question is an elongated
gland situated at the base of the pharynx, and against which the ends of the



SEA-SQUIRTS. 571

gill-bars abut. The only part of the salpa that is not transparent is the thick
mass of viscera (e, c) at the hinder end of the body ; while the muscular bands,
by the contraction of which the water is driven through the barrel, may be
compared to the hoops of the latter. Externally the w T hole animal is invested
with a thick, tough, transparent tunic ; and in some forms there are two tail-like
appendages to the hinder end of the body. Such is the structure of a salpa ; but
there are two generations in the life of these creatures, namely, the simple form,
and the chain-like or aggregate form ; the first being shown in the upper, and the
second in the lower figure of the Plate facing p. 561. It will be observed that
in the chain the individuals are attached to one another by their upper and lower
surfaces, and thus have these two apertures free ; and when taken from the water
the whole chain, which is several feet in length, can be easily resolved into its
component units. The specimen represented in the annexed figure is one of these
detached units from a chain, the projection marked g being for the purpose of
attachment to the neighbouring individual. Although extremely interesting and
curious, the whole history of the development of salpse is so complicated that it is
almost impossible to explain it fully in a popular work. It may be stated,
however, that the solitary salpa is born from an egg carried within the body of
one unit of the aggregate form, the embryo being nourished by means of a placenta
from the blood of the parent. On the other hand, the chain-salpee are produced
asexually by budding from a stolon within the body of the solitary form. In the
chain-salpa the eggs arise, however, at an exceedingly early period of its develop-
ment, with the curious result that three generations are present at one time in a
single individual. Thus a solitary salpa has within it the buds of an aggregate
salpa, the units of which may each contain eggs which will ultimately develop
into the next solitary form. And, as a matter of fact, in a solitary salpa the
germ-cells of the embryo of the next solitary form are actually visible before the
development of the stolon which is to give rise to the chain-form. As the stolon
forms in the body of the latter, it includes within it the mass of germinal cells ;
and while the former elongates to form the chain of units, the mass of germ-cells
likewise lengthens, with the result that a single egg-cell is shut off in each unit
of the chain. Simple salpas vary in size from a quarter of an inch to upwards of
eight inches ; and in some parts of the ocean-surface are met with in incalculable
swarms. Although more abundant in tropical than in the cooler seas, their
northward range extends beyond Scotland and Norway, while to the south they
have been taken below the latitude of Cape Horn and the most southern point of
Australia. Dr. Brooks writes that " they are abundant only after the water has
been for some time undisturbed by winds; and as prolonged calms are most
frequent in warm seas, those waters are most favourable for the development of
these animals, which multiply with most astonishing rapidity. The smaller species
are often so abundant that for hundreds of miles any bucketful of water dipped up
at random, will be found to contain hundreds of them. In such places collecting
with the surface-net becomes impracticable, for almost as soon as the net is dropped
into the water, it becomes choked with a mass so dense that nothing can enter it."
The food of these creatures consists of minute marine organisms, both animal and
vegetable. In swimming, chain-salpae progress by an undulating, snake-like move-



572 SEMIVERTEBRATES.

ment. Usually, the family is divided into the two genera Salpa and Cydosalpa, the
latter being distinguished by having the digestive tract coiled up; but some writers
have divided the first of these two into several subgeneric groups. A second family
is represented by the very imperfectly known genus Octacnemus, dredged at depths
of between one and two thousand fathoms in the South Pacific ; the body being
much flattened, and probably attached by one extremity. Nothing is known as
to the life-history of this singular form.

The second suborder Cyclomyaria of the free-swimming non-luminous as-
cidians takes its name from the muscular bands of the inner tunic forming perfect
rings, and is typically represented by the genus Doliolum. The life-history is
complicated by polymorphism ; the tailed larva developing into a sexless form,
the buds from which give rise to nutritive units, fostering units, and reproductive
units. In the typical genus all the muscles form encircling hoops, and the three
forms of the sexual generation occur together on one stolon, or outgrowth ; but in
Anchinia there are only two complete muscular rings, and the three forms of the
sexual generation are produced successively.

The free-swimming form known as Appendicularia is the type
Tailed Ascidians.

of the third and last order Larvacea of the class, all the members

of which are characterised by the possession in the adult state of large tail-like
appendages, furnished with a skeletal axis. These creatures, which are of minute
size, have not undergone the degeneration so noticeable in the adult of the other
tunicates, and thus correspond much more closely to the larval stage of the latter.
A curious feature is the rapid production of a temporary outer tunic, which may
be shed at any time, and replaced by a second one. There is no separate atrial
cavity; and the branchial chamber is simply an elongated pharynx, with two
openings on the lower surface, which correspond to the gill-slits, and are well
furnished with cilia. The nervous system consists of a large ganglion placed in
the anterior part of the dorsal surface, followed by a long chord, provided
with smaller ganglia, and extending backwards over the intestine to reach the
tail, where it runs along the left side of the skeletal axis. The intestine itself is
situated behind the branchial chamber, and the vent opens on the inferior or
ventral aspect of the body in advance of the gill-slits. Neither budding, meta-
morphosis, nor alternation of generations takes place ; and the reproductive organs
are situated at the hinder end of the body. The group comprises only the single
family AppendiculariidcK, which contains five genera, the names and characters
of which it will be unnecessary to mention.



Online LibraryRichard LydekkerThe new natural history (Volume 5) → online text (page 60 of 62)