Part 7

The remarkable _Rhopalæa neapolitana_, from Naples, may be roughly compared to an hour-glass with a very long constriction. The test is smooth in the upper part, but knobby and encrusted with foreign bodies below. The upper or thoracic end contains the branchial sac, and the lower or abdominal portion the stomach, heart, and reproductive organs, the gullet and intestine traversing the whole length of the narrow central region. Although from its general structure _Rhopalæa_ is a Clavelinid, it is not certainly known to produce buds.

_Perophora listeri_ (Fig. 15) occurs in the form of little jelly-like transparent blobs rising by short stalks from a silvery thread-like stolon. Owing to their small size and transparency, it is possible to examine specimens alive under the microscope, the currents passing through the stigmata in the walls of the branchial sac, and the beating of the heart being distinctly visible. The rapid motion of the cilia surrounding stigmata gives the appearance of dark wheels all rotating in the same direction. The heart beats so as to drive the blood current so many times in one direction, and then after a short pause, in the reverse direction.

The exhibited specimen growing on an oyster shell, is from Plymouth.

_Sub-order 2._—ASCIDIÆ COMPOSITÆ.

The Compound Ascidians are fixed forms, which give rise to colonies by budding, the individuals being immersed in a common mass and not possessing separate tests.

Although reduced to an extremely small size each individual or ascidiozooid of a colony possesses the same organs as a large Simple Ascidian, excepting that the former does not possess a separate test. Frequently the individuals of a colony are grouped into systems, in which the atrial orifices open into a common cloaca. The little ascidiozooids vary greatly in shape in the different families. In the _Polyclinidæ_, for instance, they are long, the organs being so to speak, drawn out, and being arranged in three regions, the thoracic, abdominal and post-abdominal, the first region containing the branchial sac, the second the stomach, and the third the heart and reproductive organs. In the _Distomidæ_, the body exhibits two regions, thoracic and abdominal, the heart and reproductive organs lying alongside of the stomach. The _Botryllidæ_ comprise only one region, the stomach and the other organs being situated by the side of the branchial sac.

The Compound Ascidians include seven families which are characterised chiefly by the method of bud formation, and by the arrangement of the organs into one, two, or three regions.

It is only possible, from limits of space, to refer to a few interesting forms.

The species of _Botryllus_ are those most commonly met with. They form richly coloured gelatinous incrustations on rocks and seaweeds. _B. violaceus_ (Figs. 16, 17, and 18 D) is blue with white lines; _B. smaragdus_, green; _B. marionis_, brown with white and carmine; _B. castaneus_, purple, and so on. The individuals are arranged in circular systems with the branchial orifices round the circumference and the atrial orifices opening into a common central cavity (Fig. 17), the whole colony being composed of groups of systems.

[Illustration:

Fig. 16.

_Botryllus violaceus_ on seaweed. (After H. Milne-Edwards.) ]

The exhibited specimen of _B. violaceus_ was grown in the tanks of the Biological Station at Plymouth. The red specimen of _B. aurolineatus_, from Naples, shows well the branchial and cloacal orifices. In _Botrylloides_, the individuals form elliptical or elongated systems.

_Colella thomsoni_ was obtained near the Philippines at a depth of 10 fathoms. The specimen, which is about 7 inches in length, resembles an elongated head of clover on a thickened stalk. The individuals which compose the head are arranged in spiral lines, the atrial orifice of each ascidiozooid opening separately and not into a common cloaca.

[Illustration:

Fig. 17.

A. _Botryllus violaceus_, magnified, showing two systems of 6 and 7 ascidiozooids. B. One ascidiozooid extracted.

_a_, branchial; _b_, atrial orifices; _c_, branchial sac; _d_, stomach.

(After H. Milne-Edwards.) ]

_Colella quoyi_ (Fig. 18 A), from 25 fathoms off Kerguelen Island, forms a rounded head on a short peduncle, the total height being one inch. The ascidiozooids are arranged in vertical lines in the “head,” each line consisting of a double zigzag series.

[Illustration:

Fig. 18.

Colonies of _Ascidiæ compositæ_, natural size. A. _Colella quoyi._ B. _Leptoclinum neglectum._ C. _Pharyngodictyon mirabile._ D. _Botryllus._

(After Herdman, _Challenger_ Report and Encyclopædia Britannica.) ]

_Julinia ignota_, from the Antarctic regions, forms long narrow colonies, which attain a length of nearly three feet. One end is attached, the rest of the colony apparently lying along the sea-bottom.

_Amaroucium roseum_ from Naples forms translucent gelatinous masses; a slice is exhibited, showing the long slender ascidiozooids immersed in the mass.

[Illustration:

Fig. 19.

_Pyrosoma elegans_, natural size. A. Side view of entire colony. B. End view of open extremity.

(Herdman: _Tunicata_, Encyclopædia Britannica.) ]

_Pharyngodictyon mirabile_ (Fig. 18 C), from 1600 fathoms in the Southern Indian Ocean, resembles a small mushroom, and is about one inch in height. This species is one of the few deep-sea Compound Ascidians.

_Leptoclinum albidum_ is a common and widely distributed species; it occurs in the form of thin white crusts. The glistening white appearance is due to the common test being densely crowded with minute stellate spicules of carbonate of lime.

The specimen of _Leptoclinum neglectum_ (Fig. 18 B) encrusts a fragment of sponge.

_Goodsiria pedunculata_ from the Straits of Magellan, forms a rounded cartilaginous mass attached by a short peduncle; sometimes several masses are attached to each other. Each of the small dark oval areas on the surface corresponds to the branchial and atrial orifices of one ascidiozooid.

_Sub-order 3._—ASCIDÆ SALPIFORMES.

The Salpiform Ascidians comprise only one genus, _Pyrosoma_, which occurs in the form of free-swimming colonies shaped like hollow cylinders closed and rounded at one end and open and truncate at the other (Fig. 19). The wall of the cylinder is formed of a single layer of ascidiozooids (Fig. 20), so arranged that all the atrial orifices open into the interior of the cylinder, and all the branchial orifices on the exterior, the two kinds of orifices being at opposite ends of the body, and not close together, as in most simple and compound Ascidians.

Specimens vary in size from a few inches to upwards of four feet in length, and, as the name of the genus implies,[22] they are brilliantly phosphorescent. Sometimes they occur in innumerable multitudes, giving rise to a zone of greenish light extending for miles. Professor Moseley records that during the voyage of the _Challenger_ in the North Atlantic a huge specimen of _Pyrosoma spinosum_, four feet in length, was captured. On tracing his name on its body, the word came out in letters of fire.

[Illustration:

Fig. 20.

Section through wall of _Pyrosoma_, magnified, showing a single layer of ascidiozooids.

_br_, branchial; _at_, atrial orifice; _tp_, process of the test; _br s_, branchial sac.

(Herdman: _Tunicata_, Encyclopædia Britannica.) ]

In _Pyrosoma elegans_ (exhibited), from Naples, the ascidiozooids are arranged in verticils, and the mouth of the cylinder is surrounded by a movable diaphragm; the outer end of each ascidiozooid is provided with a membranous spine. Six species of _Pyrosoma_ are known. _Pyrosoma atlanticum_ is found in the tropical Atlantic and Antarctic; _P. giganteum_ in the Atlantic, Pacific, and Antarctic; and _P. spinosum_ in the South Atlantic.

Order II.—THALIACEA.

The Thaliacea are free-swimming Tunicates, which exhibit alternation of generations in their life history. There are three families, _Salpidæ_, _Octacnemidæ_, and _Doliolidæ_.

_Salpidæ._—The Salpas are transparent barrel-shaped organisms which occur in abundance at the ocean surface. They are so transparent that they are rarely seen, except in calm weather from the side of small boats; yet they frequently swarm in countless multitudes. From five to ten bands of muscles partially or entirely surround the body, like hoops. The branchial and atrial openings are at or near the opposite ends of the body. The branchial sac has almost disappeared, the dorsal lamina and ventral gutter (or endostyle) alone remaining, the interval between the two on each side representing an enormous stigma; the dorsal lamina, or “gill” is the transversely striated band passing obliquely across the body and forming the only barrier between the branchial and atrial cavities. Water enters at the mouth, and, by the contraction of the muscle-hoops, is driven out through the atrial aperture at the opposite end, which is then closed by a sphincter muscle. The elastic walls of the body expand, and water again enters through the mouth, the valve-like lips of which prevent its being driven out that way. The _Salpa_ swims along in jerks, and along with each gulp of water takes in Radiolaria, Foraminifera, etc., which are retained by the mucus of the endostyle and carried to the gullet. The _Salpa_, in fact, lives, as Professor Brooks observes, in a “living broth,” so abundant is the food supply.

The intestines usually form an oval mass termed the “nucleus,” which is a conspicuous object at the posterior end.

[Illustration:

Fig. 21.

Posterior part of solitary form of _Salpa democratica-mucronata_, showing a chain of embryos nearly ready to be set free.

_gem_, young chain of _Salpæ_; _st_, stolon; _t_, test; _visc_, visceral mass. ]

The _solitary Salpa_ above described is asexual. In the ventral region of its body it forms a stolon which becomes segmented into a series of buds (Fig. 21). As the stolon grows the end series of buds breaks off in the form of a chain and swims away, other chains being detached in succession. A chain is formed of individuals arranged in two rows, the individuals in each row being alternate (not opposite).

Each individual of a chain differs from the solitary individual in shape, arrangement of muscle bands, etc., but especially in having reproductive organs. The chain Salpid is hermaphrodite; the embryo develops into a solitary asexual _Salpa_ which produces the chains by budding. The wonderful life history of _Salpa_ was discovered by the poet Chamisso during a voyage round the world in 1819. He observes: “A _Salpa_ mother is not like its daughter or its own mother, but resembles its sister, its granddaughter, and its grandmother.” Here we have an example of “alternation of generations,” a sexual generation (chain form) giving rise to an asexual generation (solitary form), which latter produces the sexual generation.[23]

Most of the species of _Salpa_ have double names owing to the chain and solitary forms having been regarded as distinct species before they were known to be phases in the life history of one and the same species. _Salpa runcinata-fusiformis_, solitary form (Fig. 22 B), is barrel-shaped, truncated at each end, with terminal orifices, and with nine muscle-bands on the dorsal surface, some of which converge towards each other. An individual of a chain (Fig. 22 A) is fusiform, with six muscle-bands, and with the orifices not terminal, but at each end of the dorsal surface.

The solitary form of _S. africana-maxima_ is barrel-shaped, with truncated ends and terminal orifices, and with nine broad parallel muscle-bands. The chain form is conical at one end, with six bands, and with orifices on the dorsal surface. The exhibited specimen of the chain form, which is in an early stage of growth, contains 202 individuals. The solitary and chain individuals of _Salpa costata-tilesii_ attain a length of six to eight inches. The solitary form has eighteen muscle-bands and two large spines at the posterior end. The individual of the chain has five muscle-bands. A chain of three individuals is exhibited.

_Salpa pinnata_ produces a circular chain; the exhibited specimen of the solitary form shows a small chain about to be detached; a circular chain of six individuals is also exhibited. Species of _Salpa_ abound in all seas, but specimens from Naples have alone been exhibited on account of their good preservation.

Family _Octacnemidæ_ includes _O. bythius_, a deep-sea Salpid, in which the body forms a flattened disk produced into eight radiating lobes.

[Illustration:

Fig. 22.

_Salpa runcinata-fusiformis_. A. Chain form. B. Solitary form. 1–9, muscle bands; _em_, embryo; _m_, mantle; _visc_, visceral mass or nucleus.

(Herdman: _Tunicata_, Encyclopædia Britannica.) ]

[Illustration:

Fig. 23.

_Doliolum denticulatum_, sexual generation, from the left side. _m^1_-_m^8_ muscle bands; _at_, atrial; _br_, branchial apertures; _br s_, branchial sac; _sg_, stigmata; _st_, stomach; _ng_, nerve ganglion; _so_, sense organs.

(After Herdman, Encyclopædia Britannica.) ]

Family _Doliolidæ_. The body is cask-shaped and surrounded by circular hoops. The branchial and atrial orifices are at the opposite ends. The branchial sac is pierced by two oblique bands of stigmata (Fig. 23 _sg_). The life history is very complicated. The egg develops into a tailed larva, which develops into a “nurse”; the latter is asexual, and produces three kinds of buds on a stolon, viz. (1) nutritive buds which provide the “nurse” with food, (2) foster forms which are set free as cask-shaped bodies with eight broad muscle-bands, and (3) sexual forms which are attached for a time to the foster forms, but which later become free and give rise to the egg.

Order III.—LARVACEA.

[Illustration:

Fig. 24.

_Oikopleura cophocerca_ in its “house” (after Fol); seen from right side, × 6. Arrows indicate course of the water; _x_, lateral reticulated parts of the “house.” ]

The Larvacea are very minute Tunicata which live at the surface and swim by means of a tail-like appendage, resembling in this and certain other respects the tadpole larva of other Tunicata. They are able to form a temporary test or “house” many times larger than the body (Fig. 24). The organism itself, which is almost lost in its large test, is the little hammer-shaped body in the centre of the figure; the streaked areas bound a space in which the tail lashes vigorously. The animal can leave its test and secrete another in a few hours.

The tail is attached to the under or ventral surface of the tiny little barrel-shaped body, and usually points forwards; a skeletal rod, the urochord, runs along its length. The branchial sac has two ciliated openings or gill-clefts leading directly to the exterior, and not opening like the stigmata of the other orders into an atrial cavity.

The order contains one family, the _Appendiculariidæ_, and four genera, and is represented in all seas.

_Oikopleura cophocerca_, one of the largest forms, is about half an inch in length. The exhibited specimens came from St. Andrews, Fife. Professor McIntosh reports that occasionally specimens of this species occur in immense quantities, the tow-nets being filled with them.

THE STARFISH GALLERY.

In the STARFISH GALLERY is exhibited a series of the animals belonging to the class _Echinoderma_; of these the Starfishes are the best known, while others are the Sea-Lilies, Sea-Urchins, and Sea-Cucumbers or Sea-Slugs.

A small collection of various kinds of Worms is also exhibited in this Gallery (Wall-cases I.–III.).

ECHINODERMA.

Six table-cases contain the dried Echinoderms arranged in systematic order. The seventh is devoted to preparations, models, and figures illustrative of the structure and life history of various members of the group.

An inspection of that Case and the accompanying woodcuts will make clear the distinctive characters of the Echinoderma. Unlike that of a Crayfish or a Mussel, the body does not appear to be divided into two equal or symmetrical halves, though it really is; this is due to the possession of a number of rays, of which there are ordinarily five. The skin is strengthened by the deposition in it of carbonate of lime, which may be in the form of continuous plates or bars, or of separate scattered spicules. A series of tube-feet or suckers (podia) are generally developed along each ray, and these are supplied by a system of water-vessels peculiar to Starfish and their allies. These rays are often called “_ambulacra_.”

[Illustration:

Fig. 1.

A. Anchor and plate of _Synapta_. B, C. Tables of _Holothuria impatiens_; and D. _Holothuria atra_: from various aspects. E. Spicule from sucker of _Stichopus variegatus_, magnified about 200 times. ]

[Illustration:

Fig. 2.

Diagram of Water-vessels.

_c.c._ Circular canal, with _p.v._, its Polian vesicles; from it a radial canal (_v.c._) is given off along the lower surface of each arm; this supplies, by side branches, the suckers, _s_; connected with each sucker is a contractile swelling or ampulla (_a_). The circular canal is in connection with the exterior by _s.c._, the stone-canal, and opens to it by the madreporite (_m_). ]

[Illustration:

Fig. 3.

Figure of a Starfish (_Asterias rubens_).

In the ray marked I. the skin has been removed from the upper surface, and the ambulacral ossicles (_ao_) and the podia (_s_) are seen _in situ_; the blind outgrowths (_c_) from the central stomach (_sp_) have been dissected out. In II. the gonads (_g_) are exposed; and in the centre above the stomach the rectal glands (_rg_) are to be seen. The anus (_a_) is seen to be subcentral in position. ]

In the body of the Starfish (Fig. 3) the arms are seen to be continuous with the disk and to contain portions or prolongations of the chief organs. The middle of the arm is occupied by two rows of hard pieces (ambulacral ossicles), the fellows of which make an open angle with each other, and so form an open ambulacral groove; along this we find the suckers, the water-canal that supplies them, the blood-vessel of the arm, and a nerve-cord. At the centre of the disk is the mouth. The ossicles at the sides of the arms bear spines, which vary in different species; the surface of the back is supported by a network of hard pieces, and through the intervening spaces there project membranous pouches, which are respiratory in function. The modified plate on the upper surface opens into a tube by means of which the water-vessels communicate with the exterior; this plate is known as the madreporite (Fig. 2, _m_).

The organs for masticating the food are most highly developed in the regular Echinoids, where the complex apparatus known as the “Lantern of Aristotle” is found (Case 38) to consist of five sets of pieces; the tooth is strong and bevelled at its free end; it is supported by triangular jaws on either side, a pair uniting and having the form of an inverted pyramid; these alveoli are connected with their neighbours by oblong pieces (_falces_); above these there are elongated bars, which are hinged on to the inner end of the falces and have their outer ends free. The whole lantern is connected to the test by muscles which pass from its sides to the auricles or upstanding pillars which lie round the mouth; and, owing to this muscular apparatus, the teeth are capable of complicated and various movements.

In the Ophiuroids the edges of the mouth-slits are provided with short spinous processes, varying a good deal in arrangement, but never having, apparently, any other function than that of a filtering-apparatus; in the Starfishes the plates round the mouth have a supporting function only; in Crinoids and Holothurians the mouth is unarmed; the latter are often remarkable for a deposit of calcareous plates in the walls of the gullet, and in the former the grooves on the arms are the lines along which food comes to the mouth.

Echinoids live on seaweeds and the animals that are found on them; such as have no teeth, like _Spatangus_ (Case 32), use their spout-like mouth to take up the sand and débris on which they move, and from which they extract some nutriment. Ophiuroids live on the smaller foraminifera; Asteroids on dead fishes (as line-fishermen well know), oysters, and other molluscs, and even on specimens of their own particular species; Holothurians on shell or coral débris and the minute organisms it contains; and Crinoids on small tests of foraminifera and on the adults of small and larvæ of larger crustacea.

In a number of Echinoids and Asteroids some of the spines are specially modified to act as seizing-organs—the free end being divided into two, three, or rarely four pieces, which are moved on one another by special muscles. These minute organs were regarded by earlier observers as parasites, and were named _pedicellariæ_; they may be movable, when they have a stalk, or the stalk may be absent and the valves sessile. Considerable difficulty attaches to the determination of the use that these organs may be to their possessors; but there is reason to suppose that they may act as cleansing-organs by removing minute particles of dirt, and as temporary organs of fixation, while M. Prouho has observed their use as organs of defence.

Echinoderms move but little; the unstalked Crinoids, if they cannot find stones or worm-tubes around which to attach themselves, swim by beating the water with their delicate arms, five being raised and five depressed alternately. The Echinoid or Asteroid is able to move by the aid of its podia or so-called ambulacral feet, which become erected by being filled with water, and are then contracted; by means of this contraction movement is effected; a similar kind of locomotion obtains with the pedate Holothurians; in the Ophiuroids the flexible arms either serve as the organs of movement, or act as an apparatus whereby the creature becomes coiled round the branches of corals (see Case 20).

Echinoderms are often of exceedingly bright colours, as is shown by the pictures on the wall, and are very conspicuous objects; this may, apparently, be associated with disagreeable tastes or odours; sometimes they cover themselves over with seaweed, and so hide their brilliancy; the spines of some forms are exceedingly painful to the touch, and the stout plates of some of the _Goniasters_ must form admirable organs of protection. The power of restoring lost or injured parts is one of the most remarkable points in the Echinoderm organization (see Case 6).

Echinoderms are of great geological age, and were very abundant in earlier periods of the world’s history. Two groups (the Blastoids and Cystids) have completely disappeared, and the Stalked Crinoids (Lily-Encrinites) are far less common than they used to be. Echinoderms are now found in all seas, and extend to great depths of ocean; many of the species have exceedingly wide areas of distribution, and most are characterized by their gregarious habits, a large number of specimens of a single species being generally obtained by the dredge. They are most abundant in the tropical seas.

Most Echinoderms lay their eggs in the water, where the larvæ are developed and swim about freely; but in a few (_Hemiaster_, _Ophiacantha vivipara_, and others) the young do not pass through any metamorphosis, for the eggs are placed in special pouches of the body of the parent, in which they are hatched. The free-swimming larvæ of the other Echinoderms pass through a series of remarkable changes (Figs. 4 and 5); these are illustrated by the twelve models of various forms of larvæ exhibited in Case 36; in Case 35 is a set of models showing in detail the changes undergone by a single species (_Asterina gibbosa_). A portion only of the body of the larva is converted into the substance of the perfect animal; the rest is either absorbed by the growing animal, or shrivels up and disappears.

[Illustration: Fig. 4. _Pluteus._ Fig. 5. _Bipinnaria._ Developing larvæ.]

Below the twelve models in Case 36 may be seen a representation of three stages in the history of the Feather-star (_Antedon bifida_). The larvæ of this Echinoderm are not free, but are attached by a stalk (Fig. 6); in the common Feather-star and other _Comatulidæ_ the stalk is found during larval stages only; in others, such as _Pentacrinus_, it persists throughout life.

The presence or absence of this stalk has been taken as the first character of importance in the classification of Echinoderma which may be divided into two groups:—

A. PELMATOZOA,[24] or Echinoderms provided with a stalk throughout life or in the larval stages only. To this group belong the _Crinoidea_, and the extinct _Blastoidea_, and _Cystidea_.

[Illustration:

Fig. 6.

Pentacrinoid stage of _Antedon rosacea_.