Part 5

The mouth is situated in the centre of the lophophore, surrounded by the circle of tentacles; and the latter, by the action of their cilia, set up currents which convey food to the mouth.

The mouth leads into a pharynx and gullet, the latter opening into a stomach, whence the intestine ascends to terminate in the vent opening below and outside the circle of tentacles; the intestines, in fact, form a U-shaped tube (Figs. 4, 5) suspended in the body-cavity in the interior of the cell. A cord, the funiculus, passes from the stomach to the base of the body-cavity. A small nerve ganglion is situated within the upper part of the loop of intestine.

The tentacles, intestines, and other organs constitute the “polypide,” the cell being simply the protective house formed by the latter.

The body-cavity, which contains fluid, is in direct communication with the interior of the tentacles, which are hollow, and which act as respiratory organs by bringing the fluids of the body-cavity in proximity to the water. In _Flustra_ the body-cavities of the cells are shut off from each other, but pores and sieves in the partition walls allow of the junction of the inner linings of these cavities. The male and female reproductive elements are formed in the body-cavity. The egg develops in a helmet-shaped brood-pouch, the ovicell, situated at the summit of the cell and almost immersed in the cell above. The ciliated embryo swims about for a few hours and settles down to form the first polypide and cell; from the latter there arise buds which remain attached, and produce other buds, till a colony like that of _Flustra_ results.

Among the ordinary cells are certain smaller cells (Fig. 1, _a_) slightly raised above the general level, different in shape from the ordinary kind and with thicker lids. These peculiar cells are termed avicularia, and chiefly contain muscles for opening and shutting the lid. They arise by modification of the ordinary cells, whereby all the organs of the polypide have become atrophied except the muscles. The Polyzoa[12] were so named by Vaughan Thompson, who, in 1820, discovered that certain plant-like animals, which had previously been classed with the zoophytes, possessed a much higher organisation, in that the intestine was separate from the body-cavity and not continuous with it as in Sea-Firs, Sea-Anemones, and Corals. In 1834, Ehrenberg named the group Bryozoa[13] or Moss Animals.

With the exception of one genus (_Loxosoma_), all Polyzoa form colonies, which arise by the continual budding of the cells, the buds remaining attached to the parent cells. The colonies vary endlessly in form and habit, occurring as crusts on rocks, etc., masses, broad fronds, branching tree-like growths, bushy tufts, etc.

The texture and consistency may be gelatinous, cartilaginous, horny and flexible, or stony.

The great majority of species are marine, but a considerable number inhabit fresh water. The Polyzoa are classified as follows:—

{Sub-order 1. Chilostomata.[15] {Order I. Gymnolæmata.[16] { Orifice of cell with a { Lophophore and tentacular { horny lid. { crown circular. { { Without a lobe over {Sub-order 2. Ctenostomata.[17] { the mouth. { Orifice of cell closed { { by a membranous Group I. Ectoprocta.[14] { { comb-like frill. Vent opens outside {Order II. Phylactolæmata.[18] { Always fleshy or the circle of tentacles.{ Lophophore and tentacular { horny. { crown horse-shoe { { shaped. With {Sub-order 3. Cyclostomata.[19] { lobe over the mouth. { Without lid or frill; { Fresh water forms. { orifice of cell usually { circular; cells { always calcareous.

Group II. Entoprocta.[20] Vent opens inside circle of tentacles.

_Sub-order 1._—CHILOSTOMATA.

[Sidenote: Cases A and B 1.]

The Chilostomata, which contain many more species than all the other groups put together, are divided into three sections:—_A._ Cellularina, in which the cells are more or less boat-shaped or cornucopia-shaped, and joined together to form flexible branching colonies; _B._ Flustrina, in which the cells are typically shaped like oblong boxes with membranous front walls; and _C._ Escharina, in which the whole front wall is calcified.

[Sidenote: Case A 1.]

[Illustration:

Fig. 6.

A, _Bugula turbinata_, natural size, B, portion × 50.

_a_, avicularia; _m_, mouth; _o_, ovicell.

[‘The Cambridge Natural History.’] ]

_Section A._ CELLULARINA.—_Bugula turbinata_, or the Bird’s-head Coralline (Fig. 6) grows attached to rocks near low water mark in the form of spiral tufts about two inches in height, composed of narrow flat branches in which the cells are arranged from two to six abreast and all facing upwards. Each cell is boat-shaped and with nearly the whole front surface membranous; the globular bodies at the head of certain cells are the ovicells. Attached to the outer edge of each cell is a remarkable object resembling a bird’s head, and hence termed avicularium, seated on a short stalk. The head and beak contain powerful muscles for opening and shutting a horny lid or mandible hinged on below. In life, the avicularium sways to and fro on its stalk, with the lower “jaw” continually snapping up and down in the most ludicrous fashion. The beak is capable of seizing and holding quite large objects.

The function of these curious appendages is partly to warn off trespassers and partly to capture and retain small animals till decomposition has set in; in the latter case, the currents set up by the tentacles draw in the particles to the mouths of the polypides. The avicularia have arisen by modification of the ordinary cells, in which the muscles have developed at the expense of the degenerated polypides, the cells have become much smaller, of different shape, and separated out from the rest; the mandible represents the lid or operculum of the ordinary cell. The avicularia vary greatly in size and shape in the different genera; in _Flustra_, for instance, these organs closely resemble the ordinary cells.

[Illustration:

Fig. 7.

_Bugula bicornis._ Cells magnified. (After Busk.) ]

[Sidenote: Case A. Upright part.]

In _Bugula bicornis_(*) (Fig. 7), from 1950 fathoms in the Southern Indian Ocean, each cell is provided with two avicularia with remarkably long stalks. The graceful vase-shaped _Kinetoskias cyathus_(*) (Fig. 8), one of the treasures of the “Challenger” Expedition, was dredged from 1525 fathoms off Cape St. Vincent. The stem, which tapers gradually upwards, rises from a tuft of root fibres. The cup is formed of slender branches supported at the base by a delicate membrane. The branches are composed of biserial rows of cells (Fig. 9) opening towards the interior of the cup. The avicularia are pear-shaped and pedunclate. Probably, in life, the cup is capable of being opened out to a considerable extent. Specimens of this species were also obtained from 2160 fathoms in the South Atlantic.

[Illustration:

Fig. 9.

_Kinetoskias cyathus._ A branch magnified.

_a_, an avicularium. (After Busk.) ]

[Sidenote: Case A 1.]

_Scrupocellaria reptans_, or the Creeping Coralline (Fig. 10 A, B) forms branching colonies, creeping over rocks and seaweeds, and attached by horny fibres often provided with curved hooks. The branches are composed of cells arranged in a double row. Each cell has the membranous area of its front surface protected by a branched flattened spine or operculum, and is produced and narrowed below; at the upper outer margin is a minute triangular avicularium. At the base of the back surface is a small sack-shaped cell with a cleft at the upper end, in which a horny bristle is articulated. The little cell is termed a vibracular cell, and the bristle a vibraculum.[21] This organ has arisen by a further modification of an avicularium, whereby the horny lid of the latter has become a long bristle. The bristles by their motion keep off intruders, and possibly act as scavengers by sweeping the surface of the cells.

[Illustration:

Fig. 8.

_Kinetoskias cyathus._ (From Voy. Challenger, Atlantic: Wyv. Thomson.) ]

[Illustration:

Fig. 10.

_Scrupocellaria reptans._ A. Creeping over seaweed, natural size; B. Front surface, magnified.

_a_, branched spine covering front of membranous area; _b_, avicularium; _c_, vibraculum.

C. Back surface; _a_, vibracular cell; _b_, vibraculum. ]

In _Caberia ellisii_ the vibracular cells are very large. The vibracula, which are long and serrated, have been observed to move in unison like a double row of oars.

[Sidenote: Case A 1, 2.]

_Section B._ FLUSTRINA.—In this group the colonies form leafy lamellæ, crusts, etc., in which the individual cells are typically in the form of oblong boxes with their front walls wholly or partly membranous. _Flustra foliacea_ has already been described. [Sidenote: Case A 1.] In _Flustra carbasea_ the fronds are formed of only one layer of cells, and not of two layers back to back as in _F. foliacea_. The fine specimen of _Flustra nobilis_ from S. Africa is so called from the large size of its long hexagonal cells which form a honeycomb pattern clearly visible to the naked eye.

[Sidenote: Case A 1.]

In _Flustra cribriformis_(*) (Fig. 11), from Torres Straits, the fenestrated frond forms a beautiful spiral. _Flustra florea_, from S. Australia, grows in the form of branching tufts of narrow spiral fronds. _Electra pilosa_ [dry and spirit specimens exhibited] (Fig. 12) forms a delicate silvery lace-work, encrusting shells and seaweeds (especially red algæ) on almost every shore. The long horny spine at the base of the membranous area of each cell gives the crust a pilose appearance. In _Electra verticillata_ from West Africa, the cells form an elegant branched colony, the branches being composed of regular verticils of cells.

[Illustration:

Fig. 11.

_Flustra cribriformis._ ]

[Illustration:

Fig. 12.

_Electra pilosa._ A, incrusting a seaweed, natural size; B, cells magnified; _a_, lid or operculum. ]

[Sidenote: Case A 1.]

_Membranipora membranacea_ occurs in the form of horny incrustations on bladder-wrack, which, owing to their flexibility, are able to adapt themselves to the swaying of the fronds of the Fucus.

The _Selenariidae_ (Case B 2) form free colonies, usually orbicular in shape, convex above and concave below. In _Lunulites capulus_ alternating rows of cells and vibracula radiate from the centre of the colony.

_Section C._ ESCHARINA.—In this group, the front walls of the cells are wholly calcareous. Many species form patches or crusts on shells etc., and hence the name of the section; other species, again, form stony tree-like growths, or thick plates. Frequently one and the same species occurs in the form of crusts or of erect lamellæ, the identity being recognised by the characters of the individual cells.

Often a large number of species may be found on one shell. Two good examples of this are exhibited in Case A 2.

[Sidenote: Case A 2.]

_Lepralia pallasiana_ (Fig. 13) forms sub-circular vitreous patches on stones and shells; the cells are rather large, broadly oval, and with the front wall punctured with pores; the aperture is squarish and with a slight indentation on each side.

[Illustration:

Fig. 13.

_Lepralia pallasiana_, incrusting a shell. A, natural size; B, cells magnified. ]

_Lepralia foliacea_ forms a massive coral-like growth composed of thin contorted plates which fuse to form labyrinthine cavities, the plates being constructed of a double layer of cells back to back. A large specimen from the English Channel is exhibited in Case B, upright part. In _Lepralia_ the orifice and lid of the cell have a straight lower margin, but one large group, _Myriozoidæ_, is characterised by having a notch in the lower margin of the orifice, (Fig. 14, _Schizoporella unicornis_).

[Sidenote: Case B 1.]

In many of the Escharina, the front wall of the cell is produced into a stout process or mucro at the lower margin of the orifice (genus _Mucronella_), or, again, a collar or tube grows up round the primary orifice, thus giving rise to a secondary orifice (_Smittia_, _Porella_, etc., Case B 1).

[Illustration:

Fig. 14.

_Schizoporella unicornis_, magnified. ]

[Illustration:

Fig. 15.

_Retepora beaniana._ ]

[Sidenote: Case B 1.]

In the _Celleporidæ_, (Case B 1) the cells are typically pitcher-shaped and arranged vertically, and tend to be heaped up from the overcrowding.

_Cellepora pumicosa_ forms thick pumice-like masses composed of succeeding layers of cells. The _Reteporidæ_ (Case B 1) form delicate stony networks. The reticulate fronds may be expanded out, or may form tubular or contorted growths (Fig. 15, _Retepora beaniana_). The beautiful _Retepora phœnicea_ from Torres Straits is of a rich purple colour.

The _Adeonidæ_ form thick fenestrated plates which unite to form cavernous masses usually attached to rocks by a thick jointed stem. Several very fine examples from Port Phillip, Victoria, are exhibited in the upright part of Case B.

[Sidenote: Case A 2.]

The _Catenicellidæ_ are represented by a fine series of specimens from Australia. The colonies form dense clusters of finely beaded branches. The cells are arranged in single series, each cell being united to those above and below by a horny joint. The cells are usually urn-shaped with a triangular avicularium at each upper angle, and with the front surface variously sculptured with pores or bands (Fig. 16, _Catenicella ventricosa_).

[Illustration:

Fig. 16.

_Catenicella ventricosa._ A, natural size; B, magnified. (After Busk.) ]

Sub-order 2.—CTENOSTOMATA.

[Sidenote: Case B 2, and A upright part.]

The Ctenostomata are fleshy, horny, or membranous; never calcareous. When the tentacles of a polypide are retracted into the cell, they are protected above by a membranous comb-like frill.

The cells either bud off from each other or arise as buds on a stolon or stem.

[Sidenote: Case A, upright part.]

_Alcyonidium gelatinosum_(*) (Fig. 17), so called from its resemblance to the zoophyte Alcyonium, forms fleshy translucent growths occurring in the form of nodulated branched masses, or of long finger-like growths. The species is common round our coasts where it grows attached to stones and shells near low-water mark.

[Illustration:

Fig. 17.

_Alcyonidium gelatinosum._ A, a small piece, natural size; B, the same magnified. ]

[Sidenote: Case A, upright part.]

_Amathia_ forms bushy growths composed of slender horny branches. The cells, which are cylindrical or squarish, rise from the branches in biserial rows like Pan’s pipes. In _Amathia lendigera_(*) (Fig. 18) the groups of cells are well separated from each other, but in _A. spiralis_(*) and _A. convoluta_(*) the cells form a nearly or entirely continuous series winding in a spiral round the slender stems. In _Bowerbankia imbricata_(*) the cells are clustered on the stems. _Vesicularia spinosa_(*), or the Silk Coralline, forms delicate brown tufts resembling a filamentous alga; the cells arise separately in a single series from the hollow tubular stems and are contracted at their point of attachment.

[Illustration:

Fig. 18.

_Amathia lendigera._ A, natural size; B, magnified. ]

Nearly all the Ctenostomata are marine, but a few species live in fresh water.

Sub-order 3.—CYCLOSTOMATA.

[Sidenote: Case B 2.]

In the CYCLOSTOMATA, which are all calcareous, the usually tubular zoœcia have plain circular orifices without a lid or frill closing over the retracted tentacle-sheath. There are two sections in this group, viz., Articulata, in which the cells form branching colonies, the branches being connected by horny joints; and Inarticulata, in which the colonies may be encrusting, or erect and branching, but are without joints.

The first section includes the _Crisiidæ_.

[Sidenote: Case B 2.]

_Crisia denticulata_ (Fig. 19) forms delicate white tufts, in which the flat slender branches are composed of a double row of tubular cells. The horny joints between the branches are black. The Inarticulata occur as crusts or branching growths. In _Tubulipora flabellaris_ (Fig. 20) the colonies form little fan-shaped crusts on seaweeds. _Lichenopora hispida_ forms little white disks, in which rows of tubular cells radiate from the centre. In _Idmonea_, the colony is branched, the tubular cells being arranged in parallel rows on each side of the middle line of the branch.

[Illustration:

Fig. 19.

_Crisia denticulata._ A, natural size; B, branches magnified. ]

The Cyclostomata are all marine.

[Illustration:

Fig. 20.

_Tubulipora flabellaris._

_a_, half of an incrusting colony, × 8; _b_, a few cells, × 44; _c_, a colony, natural size. ]

Order II.—PHYLACTOLÆMATA.

[Sidenote: Table Case A, upright part.]

All the forms in this group inhabit fresh water, where, in the form of creeping or erect branching growths or masses, they grow attached to freshwater plants, tree-trunks, old wood, etc.; two species are capable of slow movement from place to place. The lophophore and tentacular crown of the polypide are horseshoe-shaped. The Order owes its name to the presence of a lobe guarding the mouth.

In addition to the sexual, there is an asexual reproduction by means of peculiar internal buds termed statoblasts (Fig. 21). When the colony dies in the autumn, the liberated buds, securely protected in a horny capsule, retain their vitality till the spring; in due season the valves of the statoblast burst open, and the contents develope into a new colony. The statoblasts, which resemble small seeds, are usually provided with a ring of air cells, which act as a float, and in some species spines are present.

[Illustration:

Fig. 21.

Statoblasts of Freshwater Polyzoa. A, _Fredericella sultena_ × 38; B, _Plumatella repens_ × 38; C, _Lophopus crystallinus_ × 28; D, _Cristatella mucedo_ × 28.

[‘The Cambridge Natural History.’] ]

[Illustration:

Fig. 22.

A, _Plumatella repens_, partly free, partly incrusting stem of water-weed. B, Cells magnified. (After Allman.) ]

[Sidenote: Case A, upright part.]

_Plumatella repens_(*) (Fig. 22) forms brown branching colonies, wholly or partly adherent to the surface of leaves of water plants, old wood, etc. The individual cells are club-shaped, and about a quarter of an inch long, each cell being attached to the upper back part of the cell below; the statoblasts (Fig 21, B) are simple oval bodies with a zone of air cells.

_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.

[Illustration:

Fig. 23.

_Cristatella mucedo_, creeping over a stem of water-weed; × 6. (After Allman.)

_a_, polypides with horseshoe-shaped crown of tentacles; _b_, statoblasts seen through the tissues; _c_, muscular sole by means of which the animal creeps; _d_, stem of water-weed. ]

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.

[Illustration:

Fig. 24.

_Pedicellina cernua._ × 27.

[‘The Cambridge Natural History.’] ]

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.

[Illustration:

Fig. 1.

British Brachiopods (_Terebratula_ and _Crania_). ]

[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).

[Illustration:

Fig. 2.

_Magellania flavescens._ Australia. Interior of valves.

A. Peduncle valve: _f_, foramen for peduncle, below which are the two small deltidial plates; _t_, hinge-teeth; _a_, _b_, _c_, muscle scars. B. Brachial valve, showing the reflected loop for support of the “arms.” ]

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).