Part 6

_Plumatella_ (_Alcyonella_) _fungosa_ forms thick masses, composed of closely packed vertical tubes. A small specimen(*) surrounding a stick from Hampstead Ponds is exhibited.

_Lophopus crystallinus_(*) occurs in the form of translucent gelatinous blobs, often attached to the slender stems of duck-weed. The statoblasts (Fig. 21, C) are elliptical and pointed at each end. The polypides are comparatively large, and can be easily observed through the transparent surface. When its delicate plumes are fully expanded, _Lophopus_ forms a beautiful object.

_Cristatella mucedo_(*) (Fig. 23) occurs in the form of greenish translucent oval or worm-like colonies with the polypides on the convex upper surface. The animal slowly creeps about on its flattened under surface.

Freshwater Polyzoa usually prefer dark places, but _Cristatella_ creeps along on the stones and pebbles in clear water, and in the sunlight. The polypides form three or more concentric rows on the upper surface. The statoblasts (Fig. 21, D) are circular, provided with a zone of air cells, and with hooked spines, the total diameter being about ¹⁄₃₀ of an inch.

Sub-class II.—ENTOPROCTA.

[Sidenote: Table Case A, upright part.]

In this small group, both orifices of the alimentary canal open within the circle of tentacles, and there is no tentacular sheath.

The polypides are borne on contractile stalks. In the _Pedicellinidæ_ the stalks arise from a creeping stolon. In _Pedicellina cernua_(*) (Fig. 24) a stolon, creeping over seaweeds, etc., gives rise to stalked cups, the movements of which are vigorous: “the polypides, when excited, dash themselves vehemently from side to side. The heads are easily knocked off, but the decapitated stalks develop fresh ones. In _Ascopodaria_ the stalks are swollen at the base; _A. fruticosa_(*), from Port Phillip, Victoria, forms beautiful tree-like colonies. The _Loxosomidæ_ do not form colonies, owing to the buds becoming detached from the parent. The species of _Loxosoma_ are always found associated with some other animal, such as a worm or Tunicate. The tentacles of the polypide are arranged obliquely to the long axis of the body, hence the name of the family (_loxos_, oblique). _Loxosoma phascolosomatum_(*) occurs, in the form of delicate tufts, on the caudal end of the Sipunculid worm _Phascolosoma_. The individuals resemble pins with little white heads, and are capable of vigorous movements to and fro; occasionally a stalk coils itself up into a spiral.

BRACHIOPODA.

[Sidenote: Small Table Case A against the west wall to left of main entrance.]

The Brachiopoda, though presenting a certain outward resemblance to bivalved Mollusca, are quite distinct from this group. They are all marine, and all possess a bivalve shell. They grow attached to rocks (Fig. 1), usually by a horny peduncle or stalk passing between the two valves, or through a foramen in one of the latter; or, peduncle and foramen may be absent, one of the valves adhering by its surface to the rocks; some species of _Lingula_ live in tubes in the sand or mud. They occur at all depths, from shallow water up to 2900 fathoms, but the largest number of species live at a depth of about 350 fathoms. Though found in all seas, the localities whence they have been obtained are comparatively few in number; but specimens are usually congregated in considerable numbers, in places where they do occur. The surviving species of Brachiopods constitute only a small remnant of a group that flourished abundantly in former epochs. There are about 150 recent, and over 6000 fossil species.

THE SHELL.—The valves of a Brachiopod shell differ from each other in size and shape, but each valve is in itself symmetrical, _i.e._, similar on each side of a middle line.

The valve through which the peduncle passes is termed the _peduncle_ or _ventral valve_ (Fig. 2, A), the other being the _brachial_ or _dorsal valve_. The peduncle valve, which is usually the larger and uppermost, contains the bulk of the viscera; in the higher genera, calcareous bars or loops (Fig. 2, B) attached to the inner surface of the brachial valve form a support for the “arms” of the animal. The inner surface of the valves presents certain markings and depressions where the muscles have been attached (Fig. 6).

The shell is constructed of very minute prisms of calcareous substance imbedded in an organic matrix. In _Lingula_ the shell is formed of alternating layers of horny and calcareous substance.

The shell-valves are either hingeless, or joined by a hinge in which teeth in the peduncle valve fit into sockets in the brachial valve. The Brachiopoda are primarily divided into two sections, _Inarticulata_ and _Articulata_, based on the absence or presence of a hinge.

The division into Orders is based on the relation of the peduncle to the valves in its passage between them or through one of them. In the most primitive Brachiopoda (_Lingulidæ_), the peduncle simply passes out between the valves and not through a foramen or pore in one of them; hence the group is named _Atremata_ (_a_, not, _trema_, pore). In the next group, including the families _Discinidæ_ and _Craniidæ_, the peduncle passes through a fissure in the edge of the peduncle valve, the fissure in recent forms becoming closed round to form a slit-like foramen; this group is named NEOTREMATA (_neos_, new, _trema_, pore). In the third group, PROTREMATA (_pro_, in front of, _trema_, pore), which includes the _Thecidiidæ_, the peduncle lies at the apex of a triangular fissure in the peduncle valve, and secretes a calcareous plate to fill in the gap. In the fourth group, TELOTREMATA (_telos_, final or complete, _trema_, pore), including the _Terebratulidæ_, etc., the triangular fissure in the peduncle valve is filled in by two calcareous plates termed deltidia, secreted by the edges of the mantle.

The valves are hingeless in the first two Orders (Inarticulata), and hinged in the last two (Articulata).

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

_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.

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.