Part 6

THE BODY.—The body usually occupies only a comparatively small space in the posterior or peduncle end of the shell. From each side of the body there is given off a thin expansion, the mantle which lines the inner surface of the shell. The space between the valves is termed the mantle-cavity. The mouth is situated in the centre of the front wall of the body or floor of the mantle-cavity. The front wall gives rise to a horseshoe-shaped platform surrounding the mouth and bearing on its upper edge ciliated tentacles, or cirri, which set up currents carrying food towards the mouth. In many genera the platform is produced into two coiled “arms” (Figs. 3 and 5), which fill up the mantle-cavity.

The name Brachiopoda (_brachion_, arm, _pous_, foot) was given to the group because these “arms” were supposed to be homologous with the Molluscan “foot.”

The mouth leads into a gullet, which opens into a stomach and intestine. In the more primitive forms the intestine terminates in a vent, but in the higher forms the distal end of the intestine has become atrophied, and consequently the gut ends blindly.

The body-cavity contains fluid, and is in communication with a system of sinuses in the lobes of the mantle (Fig. 5). Bands of muscles pass across from valve to valve. The peduncle consists of a horny outer sheath surrounding longitudinal and transverse bands of muscles.

[Illustration:

Fig. 3.

_Magellania flavescens._ (After Davidson.)

A. Interior of dorsal valve to show the “arms”; some of cirri removed on right side; _v_, mouth. B. Longitudinal section, with a portion of the animal. ]

The sexes are usually separate. The reproductive cells are formed in the body-cavity. The embryo swims freely for a short time before settling down and becoming fixed. The specimens exhibited in the case are arranged according to the following classification:—

Section I. INARTICULATA. │Order 1. Atremata. │Fam. _Lingulidæ_. ─────────────────────────┼──────────────────────┼────────────────────── „ │Order 2. Neotremata. │Fam. _Discinidæ_. „ │ „ │Fam. _Craniidæ_. Section II. ARTICULATA. │Order 3. Protremata. │Fam. _Thecidiidæ_. ─────────────────────────┼──────────────────────┼────────────────────── „ │Order 4. Telotremata. │Fam. _Rhynchonellidæ_. „ │ „ │Fam. _Terebratulidæ_. „ │ „ │Fam. _Terebratellidæ_.

Section I.—INARTICULATA.

Order 1.—ATREMATA. Family _Lingulidæ_.—The Lingulas possess emerald green or golden brown duck-bill-shaped shells. Having no hinge, the dead valves of dried shells easily fall apart. The peduncle, which is sometimes over six inches in length, passes between the pointed posterior borders of the valves. Dr. François gives a very interesting account of the habits of _Lingula anatina_ which he found living in the sand at Noumea, New Hebrides. The sole evidence of the animal’s existence is the presence, on the surface of the sand or mud, of a small, three-lobed slit (Fig. 4, upper figure). The tube (Fig. 4) in which the _Lingula_ lives is about four inches deep, flat in the upper half, rounded below. The walls of the upper flat portion simply consist of the sand with a surface coating of mucous secretion; but in the lower end the sand grains are agglutinated so as to form a distinct tube.

[Illustration:

Fig. 4.

_Lingula anatina_ in tubes in the sand; upper figure shows trilobed opening on surface of sand. Dotted line in lower figure indicates position in retraction. (After François.) ]

The edges of the mantle-folds are provided with setæ (bristles), which form three funnels protruding through the three lobes of the slit-like mouth of the sand-tube; currents enter by the lateral funnels and leave by the central.

On the least alarm the animal is rapidly withdrawn as far as the centre of the tube (see the dotted line of the shell in the figure), the surface slit and upper part of the tube being obliterated. Each of the arms forms a spiral with several coils (Fig. 5). The _Lingulidæ_ are of exceptional interest, in that they furnish a very remarkable example of “persistence of type.”

Shells of _Lingula_ occur in the earliest Palæozoic strata, and so closely resemble those of the present day, that often no difference can be observed either in the shape of the valves or in the muscular impressions on their inner surface (Fig. 6).

_Lingula_ occurs in the Indo-Pacific, Australia, China, Japan, and the Pacific Islands. _Glottidia_, a smaller form, with two small curved plates on the brachial and a ridge on the peduncle valve, is found on the American coast of the Pacific, and in the Atlantic.

[Illustration:

Fig. 5.

_Lingula anatina_, removed from shell, mantle reflected, coiled arms separated slightly; _a_, mouth. (Marginal setæ omitted.) Ventral aspect, three-quarter face. ]

[Illustration:

Fig. 16.

_Lingula anatina._ Interior of valves showing muscle scars. V. Peduncle valve, D. Brachial valve. ]

Order 2.—NEOTREMATA. The _Discinidæ_ includes two genera, _Discina_ and _Discinisca_ (Fig. 7), with orbicular conical shells, of horny calcareous composition; both valves are conical in the former genus, but in the latter the peduncle valve is flattened. Sometimes the embryos settle down on the parent shells, and we see a mass of shells in various stages of growth, as in the specimen of _Discinisca lamellosa_ from Peru.

[Illustration:

Fig. 7.

_Discinisca lamellosa._ Peru. (After G. Sowerby.)

A group of old and young specimens; largest showing foramen in peduncle valve, the rest showing brachial valves. ]

[Illustration:

Fig. 8.

Three specimens of _Crania anomala_ on a stone. Loch Fyne. ]

The _Craniidæ_ form small limpet-like shells (Fig. 8) closely adherent to the rocks by the whole surface of the peduncle valve; although this valve is so named, no peduncle or foramen is found in this family. The Neotremata, like the _Lingulidæ_, are remarkable examples of persistence of type, since forms very similar to the present day _Discinas_ and _Cranias_ occur in the Palæozoic, Ordovician and Silurian strata.

A piece of rock, with several specimens of _Crania anomala_ attached, is exhibited.

Section II.—ARTICULATA.

Order 3. PROTREMATA.—This group, formerly very abundant, is now almost extinct, the Family _Thecidiidæ_ representing the Order at the present day. _Thecidium mediterraneum_ (Fig. 9) forms little oval boxes about a third of an inch in length, shaped somewhat like a pear cut in half (peduncle valve), and with a semicircular lid (brachial valve) working on a hinge on the upper flat surface. The foramen and peduncle are absent; but between the pointed end of the peduncle valve and the hinge is an area filled in by a calcareous plate characteristic of the Protremata.

[Illustration:

Fig. 9.

_Thecidium mediterraneum._ A, natural size. B, section through shell. Magnified. ]

The brachial valve opens like the lid of a snuff-box, and shuts down on the least alarm with the rapidity of lightning. The peduncle valve is fixed on the rocks by its convex surface. The species is common in the Mediterranean in from 30 to 300 fathoms, and is also found in the West Indies.

Order 4. TELOTREMATA.—This group, which at the present day contains the largest number of species, includes the Lamp shells, so called from their resemblance to an ancient lamp. The valves are joined by a well-marked hinge, the peduncle passes through the peduncle valve through a foramen completed by two plates secreted by the mantle edges, and the brachial valve has attached to it a calcareous scaffolding of processes or loops for the support of the “arms.”

The shells in this group are frequently ridged. Their colour is usually white, but sometimes red or yellow; deep-sea forms are generally vitreous.

_Rhynchonella psittacea_ has a black shell with a pointed incurved beak; each of the arms forms a many coiled spiral and can be protruded beyond the shell; the brachial skeleton is comparatively small and simple, consisting of two separate processes.

In _Terebratulina_ the brachial skeleton forms a simple loop; in _Magellania_ the loop is reflected on itself (Fig. 2).

The beautiful and unique specimen of _Dyscolia wyvillii_, from 390 fathoms W. Indies, is remarkable for its size, being over two inches in length. The small vitreous specimens of _Terebratula wyvillii_ were obtained off Chili from a depth of 2160 fathoms; specimens of the same species were obtained also from a depth of 2900 fathoms in the North Pacific.

TUNICATA.

[Sidenote: Wall Case to left of main entrance to Shell Gallery.]

The Tunicata are marine animals, the majority of which live, in their adult stage, a stationary life, fixed to the rocks or sea-bottom, but a comparatively small number are free-swimming.

[Illustration:

Fig. 1.

_Ascidia mentula_ from the right side. _at_, atrial aperture; _br_, branchial aperture; _t_, test.

[After Herdman: _Tunicata_, Encyc. Britannica.] ]

They occur in the form of cartilaginous or leathery sacs, fleshy incrustations, solid fleshy masses, free-swimming, barrel-shaped animals, solitary or united into chains or hollow cylinders; or, lastly, of minute free-swimming tadpole-shaped organisms. To explain briefly the structure of a Tunicate, _Ascidia mentula_ (Fig. 1), is selected. The animal, which lives on a muddy bottom, in from five to twenty fathoms, resembles a conical sac fixed by the broader end, of grayish green colour and about 4 inches in height. At the narrower end are two orifices, one terminal—the branchial orifice or mouth, and the other a little lower—the atrial orifice: the former has eight lobes and the latter six.

When the Ascidian is undisturbed, the orifices are wide open, and currents enter by the branchial and leave by the atrial orifice. On the least alarm, the orifices close, jets of water being at the same time squirted out; hence the popular name “Sea-squirts” given to these animals.

[Illustration:

Fig. 2.

Diagrammatic section of _Ascidia_ representing the three sacs, and the branchial sac as the pharynx or throat.

_a_, branchial; and _b_, atrial orifice; _c_, tunic or test; _d_, mantle; _e_, branchial sac; _f_, gullet; _g_, stomach; _h_, anal orifice; _i_, dorsal lamina; dotted line indicates the endostyle. ]

The Ascidian is orientated as follows: hold the animal with the branchial orifice pointing forwards and the atrial upwards; the branchial orifice will be anterior and the opposite end posterior; the atrial orifice will lie on the upper or dorsal aspect, the opposite aspect being lower or ventral, and the sides right and left. The aspects, in fact, correspond with those of a vertebrate animal. A vertical section roughly shows the animal to be formed of three concentric sacs (Figs. 2, 3). The outermost, which is tough and membranous, is called the Test or Tunic, the whole group owing its name to the presence of this protective covering.

[Illustration:

Fig. 3.

Diagrammatic dissection of _A. mentula_.

_at_, atrial orifice; _br_, branchial orifice; _a_, anal orifice; _brs_, branchial sac; _dl_, dorsal lamina; _end_, endostyle; _m_, mantle; _ng_, nerve ganglion; _oea_, orifice of gullet; _pbr_, peribranchial cavity; _st_, stomach; _t_, test; _tn_, tentacles.

(After Herdman: _Tunicata_, Encyc. Britannica.) ]

The middle sac, termed the Mantle, which almost corresponds in shape to the outer, is composed of connective tissue, muscle-fibres, blood-vessels, etc.; in spirit specimens, the mantle is shrunk away from the test except at the orifices and at a point behind, where vessels enter the test.

The innermost or Branchial Sac is attached behind the branchial orifice and along the ventral edge, but otherwise hangs free in the interior, the space around and outside of the sac being termed the atrial or peribranchial cavity.

The delicate walls of the branchial sac, which resemble fine muslin, are perforated by innumerable vertical slits, termed stigmata, arranged in transverse rows (Fig. 4).

[Illustration:

Fig. 4.

_Ascidia mentula._ Part of wall of branchial sac showing stigmata. Magnified. ]

The margins of the stigmata are lined with cilia which set up currents; and the water which enters by the branchial orifice, passes through the stigmata into the atrial cavity, and thence out through the atrial orifice. The walls of the branchial sac are chiefly composed of a sieve-like meshwork of fine blood-vessels arranged in transverse and longitudinal rows. The currents of water passing through the stigmata aërate the blood in the vessels. Besides the stigmata, the branchial sac has two relatively large orifices, viz., the branchial orifice or mouth, and, at the opposite end, the opening into the gullet. The branchial sac is, in fact, a capacious throat or pharynx (Diagram Fig. 2 and Fig. 14). Inside the branchial orifice is a circle of fine tentacles, which guard the entrance to the branchial sac. The food of the animal consists of minute animal and vegetable organisms.

It may be wondered how this food is secured, seeing that the currents of water are continually passing through the sieve-like walls of the branchial sac to the exterior again. Within the branchial orifice and above the branchial sac are two circular ciliated ridges with a groove between, which is full of viscid secretion; the cilia on the ridges direct particles into the groove where they are retained by the mucus.

Passing backwards along the ventral edge of the branchial sac is a thick-lipped furrow, which appears like a rod in the thin-walled sac, and hence is called the endostyle. This organ secretes the mucus which is carried up by ciliary action to the circular groove in front of the branchial sac, and thence to the gullet along a fold or crest, termed the dorsal lamina, situated along the dorsal edge of the branchial sac.

The gullet opens into a large stomach situated posteriorly on the left side of the branchial sac. The stomach opens into the intestine, which, after forming a loop, terminates in the anal orifice or vent opening into the atrial cavity.

The tubular heart lies below the stomach, a remarkable feature in the circulation consisting in the periodic reversal of the blood current. An elongated nerve ganglion is situated between the branchial and atrial orifices.

[Illustration:

Fig. 5.

Ascidian Tadpole with part only of the tail _C_. Magnified section.

_N_, nervous system with enlarged brain in front and narrow spinal cord behind _n_; _N′_, cavity of brain; _O_, the single cerebral eye lying in the brain; _a_, auditory organ; _K_, pharynx; _d_, intestines; _o_, rudiment of mouth; _ch_, notochord or primitive backbone.

(From Gegenbaur’s ‘Elements of Comparative Anatomy.’) ]

_Ascidia mentula_ is hermaphrodite. The egg develops into a minute tadpole-like larva which swims about by means of its tail. Water entering by the mouth passes out through the gill-slits. A nerve-tube extending along the back and tail is swollen in front into a brain-vesicle; and underneath the long nerve-tube behind the brain is a stiff skeletal rod or axis—the notochord—which constitutes the rudiment of a backbone. Inside the brain are two unpaired sense organs, an eye and an organ of hearing (Fig. 5). After swimming freely for a few hours, the larva settles down head foremost and fixes itself by papillæ on the anterior end (Figs. 6, 7). Presently the tail becomes absorbed, and the posterior end of the nerve-tube, and the brain with its eye and hearing organ, undergo atrophy, the nerve ganglion of the adult alone representing the cerebrospinal axis of the larva. The branchial sac and intestines develop greatly, and growth proceeds in such a manner that the mouth is pushed round to a position opposite to the fixed area, and gradually the animal becomes the adult ascidian.

[Illustration:

Fig. 6.

Degeneration of Ascidian Tadpole to form the adult. The black pieces represent the rock or stone to which the Tadpole has fixed its head. ]

[Illustration:

Fig. 7.

Very young Ascidian with only two gill-slits.

(Figs. 6, 7, from Lankester’s ‘Degeneration.’) ]

This wonderful metamorphosis presents a striking example of DEGENERATION resulting from the adoption of a fixed mode of life. The active free-swimming larva with its brain, eye, hearing organ, and muscular tail becomes transformed into a comparatively inert sac.

[Illustration:

Fig. 8.

Tadpole of Frog and Ascidian. Surface view. (Lankester’s ‘Degeneration.’) ]

[Illustration:

Fig. 9.

Tadpole of Frog and Ascidian. Diagram representing the chief internal organs. (Lankester’s ‘Degeneration. A chapter in Darwinism.’) ]

The tadpole of an Ascidian resembles that of a frog (Figs. 8, 9), not merely superficially, but also in its general structure and mode of development. The Tunicata are now generally regarded as a degenerate offshoot from the ancestral stock of the Vertebrata, in that the larva possesses a skeletal rod (rudimentary backbone) separating the dorsally situated nerve-tube (cerebrospinal axis) from the ventrally situated intestinal tube, the existence of the cerebral eye in the Ascidian tadpole further tending to confirm the truth of this theory. Apart from a knowledge of the course of their development, Tunicata would have been classed among the Invertebrata, but the structure of the larva clearly reveals the affinities of the group to the backboned animals.

_Ascidia mentula_ belongs to the group of SIMPLE ASCIDIANS which are all fixed, and are either solitary or joined into colonies in which each individual or ascidiozooid has a distinct test of its own. In the COMPOUND ASCIDIANS, which form colonies by budding, the ascidiozooids are buried in a common investing mass and have no separate tests. In a third group, the SALPA-LIKE ASCIDIANS, the ascidiozooids are united to form free-swimming colonies shaped like hollow cylinders open at one end. The above three groups belong to one great Order—the ASCIDIACEA. A second Order, THALIACEA, includes the free-swimming _Salpa_ and _Doliolum_, which exhibit alternation of generations in their life history. A third Order LARVACEA, includes very minute free-swimming forms which possess a tail in the adult stage. There are sixteen families of Tunicata.

The following is a tabular view of Prof. Herdman’s classifications:—

Order I. Ascidiacea.│Sub-order 1. Ascidiæ Simplices, 4 Families. „ │Sub-order 2. „ Compositæ, 7 Families. „ │Sub-order 3. „ Salpiformes, 1 Family. Order II. Thaliacea │3 Families. Order III. Larvacea │1 Family.

Order I.—ASCIDIACEA.

The Ascidiacea include the great majority of species. With the exception of the one genus _Pyrosoma_, they lead a fixed or stationary life.

_Sub-order 1._—ASCIDIÆ SIMPLICES.

The Simple Ascidians are mostly solitary; in a few forms, however, colonies arise by budding from stolons, but each individual has a distinct test. The four families into which the sub-order is divided are chiefly characterised by the nature of the test, the number of lobes round the branchial and atrial orifice, and the character of the branchial sac.

In the family _Molgulidæ_ the tough membranous test is often coated with sand; the branchial aperture is six-lobed, the atrial four-lobed, the branchial sac has long folds or pleats, and the stigmata are curved or arranged in spirals.

_Molgula gigantea_, which is one of the largest of the Ascidians, and which attains a length of over thirteen inches, forms a tough conical sac; the branchial and atrial orifices at the upper end have six and four lobes respectively. The test is leathery, smooth above, but coated with sand below. The exhibited specimen, which comes from the Straits of Magellan, has several specimens of the stalked _Boltenia legumen_ attached to the lower part of the test.

The curious _Molgula oculata_ (Fig. 10) has a soft oval or rounded body coated with sand. The branchial and atrial orifices have respectively six and four lobes. Specimens grow attached to the rocks and also live free in the sand. The surface of the test is provided with hairs, which adhere to the rocks and collect particles of sand. The adhesion not being very firm, specimens are easily detached by currents and collected into heaps by the eddies; when living in the sand only the two dark orifices are visible. The sand coating has been supposed to confer protection by mimicry of the environment; but Professor Lacaze Duthiers found, much to his chagrin, that the sandy tests of his specimens were of no avail in securing them from being devoured by crabs who seemed to scent their prey from afar.

[Illustration:

Fig. 10.

_Mogula oculata._ _a_, branchial; _b_, atrial orifice. ]

In the family _Cynthiidæ_ the test is usually leathery, the branchial and atrial apertures four-lobed, and the branchial sac folded into longitudinal pleats.

The genera _Boltenia_ and _Culeolus_ include species in which the body is attached to a peduncle.

The large exhibited specimen of _Boltenia pachydermatina_ is 28 inches in length, the head being 4 and the stalk 24 inches long. The two four-lobed apertures are along one edge, the branchial being the lower; the body is marked with long deep furrows, and the stalk with transverse wrinkles. _Culeolus perlucidus_, from 1600 fathoms in the Southern Ocean, is in the form of a small pear-shaped head on a slender stalk, the total length being 4½ inches. The branchial orifice forms a transverse slit with raised lips near the stalk, the slit-like atrial orifice being near the rounded end of the body. _Culeolus moseleyi_, another slender-stalked form, was obtained from 2425 fathoms in the Central Pacific.

[Illustration:

Fig. 11.

A. _Styelopsis grossularia_ on shell. B. Tadpoles of same, × 9. _a_, branchial; _b_, atrial orifice. (B, after Sir J. Dalyell.) ]

The little Cynthiid _Styelopsis grossularia_ (Fig. 11), popularly known as the “Currant Squirter,” occurs in the form of bright red hemispherical blobs on stones and shells; when undisturbed, the branchial and atrial orifices expand and project upwards. The eggs are brilliant red in colour. Sir John Dalyell was the first to discover the tadpole form, which is about ⅒ inch long (Fig. 11, B), and to observe the tadpoles become fixed and develop into fixed Ascidians. He calls the active little swimming larvæ “Spinulæ,” from their resemblance to small pins.

The family _Ascidiidæ_ includes forms with a gelatinous or cartilaginous test; the branchial and atrial orifices usually have 8 and 6 lobes respectively; the branchial sac is without folds.

_Ascidia mentula_, described above, belongs to this family.

_Chelyosoma_ is characterised by the test forming tortoise-like horny plates on the upper surface. The exhibited specimen of _C. macleayanum_ (Fig. 12) comes from Greenland; the upper hemispherical part of the test is divided into 8 plates; the branchial and atrial orifices are situated in the joints between the plates.

[Illustration:

Fig. 12.

_Chelyosoma macleayanum_, slightly enlarged. _a_, branchial; _b_, atrial orifice. ]

The fine specimen of _Phallusia mammillata_ from Naples consists of several individuals partly fused together; the branchial and atrial orifices are wide open, and the mantle can be seen through the thick knobby translucent test.

In _Rhodosoma_ the test is modified so as to form stiff plates recalling the valves of a bivalve shell. One plate is attached to the rocks, the other closing against the first like a lid; the anterior end of the animal with its branchial and atrial orifices is visible only when the lid is open. The Mediterranean species _R. callense_ (Fig. 13) grows attached to the rocks. The little exhibited specimen is on a fragment of shell in front of a black patch. The figure shows specimens with the lid open and closed.

[Illustration:

Fig. 13.

_Rhodosoma callense_, × 10. A, “valve” open; B, shut. _a_, branchial; _b_, atrial orifice. (After Lacaze Duthiers.) ]

Family _Clavelinidæ_. The body is attached to a creeping stolon or mass of stolons, from which new individuals arise by budding. The other three families of Simple Ascidians included solitary forms, but the Clavelinidæ are social, and form colonies wherein each individual has its own test.

_Clavelina lepadiformis_ (Fig. 14) forms graceful crystal vases about an inch in height. The figure shows one individual, but usually the processes at the base extend out as stolons whence other individuals arise.

_Diazona violacea_, from Cornwall, forms beautiful purple disk-shaped colonies in which the ascidiozooids arise from a basal mass of stolons. Sometimes the ascidiozooids die down, leaving only a smooth violet pad, which in due time produces a new crop of ascidiozooids.

[Illustration:

Fig. 14.

_Clavelina lepadiformis_; diagrammatic, showing the anatomy. The oval bodies are the eggs; at lower end lies the tubular heart; the root-like processes at the base grow into stolons, whence other ascidiozooids arise. ]

[Illustration:

Fig. 15.

_Perophora listeri_; A, slightly, B, further magnified. Ascidiozooids in right, left, and lateral aspects.

_a_, branchial; _b_, atrial orifice. ]