Chapter III: Preface (3)

_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.

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.

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.

Fig. 19.

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

The Cyclostomata are all marine.

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.

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.’]
]

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.

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.

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.

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).

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).

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.

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_.