The commonest Entoproctous genus is Pedicellina, a graceful little animal, which occurs on many parts of our coast. It may often be discovered by looking carefully on the pink, jointed, calcareous alga, Corallina, which may be found growing at the edges of deep and cool rock-pools not too far above low-water mark. Its creeping stem or "stolon" is firmly attached to the surface of the seaweed, and sends off vertical stems here and there.[[539]] Each stem bears a "calyx," which is practically an individual of the colony. The stolon terminates, at one or both ends, in a growing-point (a), from which new individuals are budded off. The stalks bend from time to time in a curious spasmodic manner, by which means the calyces are moved about with an irritable and angry air. A good idea of the way in which the tentacles are folded away when the animal is disturbed may be obtained by putting the two wrists together, with the fingers spread out to represent the tentacles, the retraction of which would be represented by turning the tips of the fingers down into the space, the "vestibule," between the two palms. A delicate fold of skin growing from the edge of the calyx closes over the retracted tentacles, owing to the contraction of a sphincter muscle present in its circular edge. The body-wall is not separated from the alimentary canal by a definite body-cavity, so that there is no obvious distinction between the polypide and the zooecium. The existence of the Entoprocta is in fact a strong reason for refusing to admit that these two terms correspond with two different kinds of individuals.
Let us now imagine the condition we should have if a large and continuous cavity were developed between the alimentary canal and the body-wall. The body-wall would clearly have the general relations of a zooecium, while the alimentary canal and tentacles would obviously correspond with the polypide. The existence of the body-cavity would make it possible for the animal to retract its tentacles instead of merely turning them in. Regarded in this way, there is but little difficulty in comparing the Ectoprocta with the Entoprocta.
The calyces are deciduous, i.e. they are lost from time to time, the end of the stalk then producing a polypide-bud, which forms the vestibule and alimentary canal of a new calyx. Hence the phenomenon which may so commonly be noticed in Pedicellina of a "young head on old shoulders." The loss of the calyces may have some relation to the formation of the "brown bodies" in the Ectoprocta.
Another Entoproct, Loxosoma (Fig. 245) is remarkable for being the only Polyzoon which is not colonial. The buds, which are formed in two lateral series, break off as soon as they are mature, and at once begin to lead an independent existence. Loxosoma is further remarkable for being almost invariably found commensally with other animals, where it may occur in enormous numbers. L. phascolosomatum, common in the Channel Islands, is only found on the tip of the tail of Phascolosoma (see p. [428]), which inhabits the mud of Zostera-beds. Other species are found on the external surface of certain sponges (Tethya, Euspongia, Cacospongia); or on the outside of a compound Ascidian, Leptoclinum, which may itself be carried about as a detachable covering on the back of a crab (Dromia). Another species is found on the ventral surface of the Polychaet Aphrodite, and of its ally Hermione.
Fig. 244.—Side view of Loxosoma annelidicola Van Ben. and Hesse. × 50. (From Prouho.)
L. annelidicola, an interesting species recently investigated by Prouho,[[540]] was originally described in 1863 as a Trematode, under the name of Cyclatella. It escaped further notice until it was again found in the neighbourhood of Roscoff, in Brittany, on certain Polychaets belonging to the family Maldanidae (see p. [332]). The calyx has a very flattened form, and is borne on a short stalk, which terminates in a large attaching disc, formerly mistaken for the sucker of a Trematode. The features in which this species differs from other members of the genus are shown by M. Prouho to be correlated with its mode of life. The animal has the habit of lying flat on its back, the disc at the end of its stalk being firmly attached to the skin of the worm, and its short stalk being bent round into a curve so as to bring the calyx into a supine position, with its lophophore directed upwards. This habit, together with its flattened form, prevents it from being crushed between the worm and its tube. But without some further provision its position might be merely a source of danger. For supposing the calyx to be directed backwards in relation to the worm, a sudden backward movement of the latter into its tube might bring the Loxosoma into fatal contact with the inner surface of the tube. There would obviously not be sufficient room to turn round in a vertical plane, so as to bring the body into a position of safety, i.e. into a position in which it moves stalk first. But by a beautiful arrangement of the muscles of its stalk this movement is effected in a horizontal plane; on touching the Loxosoma with the point of a needle it would swing round in this way through 180° with "une rapidité qui étonne."
Urnatella[[541]] is a beautiful form with a segmented stalk, the stalks usually arising in pairs from a common base. It has at present only been found in fresh water in the United States.
Fig. 245.—Diagram of the structure of Loxosoma, seen from the oesophageal side. × about 70. a, Anus; b, buds; e, excretory organ; f, foot-gland; g, ganglion; gn, generative organs; o, orifice of vestibule; oe, oesophagus; s, stomach; t, retracted tentacles.