Sub-order 3.—Cyclostomata.
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.
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.
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.)
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.
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.