Coelomate Metazoa, small in size, and, with one exception, forming colonies, or zoaria, which are, as a rule, fixed. There is a cuticle, secreted by the ectoderm, usually thickened and rigid on the posterior part of the body, but flexible on the anterior part. The ectoderm is generally unilaminar. The nervous ganglion is single. The mouth and anus are approximated: both of them may lie within the lophophoral area (Entoprocta), or only the mouth does so (Ectoprocta). The digestive tract is U-shaped, its flexure ventral in Entoprocta, and, perhaps so, in Ectoprocta: it is always partially ciliated. The larva is free-swimming. Gemmation is universal.
There are two sub-classes, the Entoprocta and Ectoprocta, which are better considered apart.
Two well-known genera, both marine, are comprised in this sub-class, Loxosoma and Pcdicellina1 The body is more or less cup-shaped, and affixed by its dorsal or convex surface to a contractile flexible stem, which, in Loxosoma, is attached in turn to some foreign object by an expanded foot, or sole, but, in Pedicellina, is continuous with a branched stolon. This stolon usually creeps over other Polyzoa or Hydrozoa. A diaphragm separates the body from the stalk in Pedicellina, and the body frequently breaks off and is replaced by a successor, produced by budding from the stem. The foot of Loxosoma contains a foot-gland perhaps homologous with the primitive shell-gland of Mollusca2. It is always present in the young organism, and is retained in many species by the adult. It appears to be represented in the larva of Pedicellina. The rim of the cup, or body, i.e. the lophophore, is contractile, and can be partially closed over the cavity of the cup or vestibule. Just within its border is inserted a bilaterally symmetrical and single series of tentacles, ciliated on their adoral surface, towards which they can be rolled up when the animal is irritated or alarmed.
A small lobe, or epistome, which carries a tuft of cilia in the larva oi Pedicellina, projects between mouth and anus, both of which lie within the vestibule. The nervous system consists of a single ganglion, placed transversely between the mouth and anus. It gives off nerves to each of the tentacles and, in Loxosoma, to a pair of sensory tubercles ( = the dorsal organ described below ?). The nerves terminate in bipolar ganglion cells, one branch of which goes to an ectodermic sense cell provided with a single sense-hair. In Loxosoma each tentacle-nerve has a ganglion at the base of the tentacle. Sense cells are found on the aboral surfaces of the tentacles and, in Loxosoma, numerously round the rim of the vestibule, more sparsely elsewhere, and rarely on the stem. The intestine is U-shaped, and consists of a stomodaeum and a mesenteron, the cells of which contain pigment. There is a proctodaeum in Pedicellina, but in Loxosoma it is possible that the blastopore persists as the anus. There is no body cavity, but a gelatinous matrix with connective tissue cells and muscle cells occupies the interval between the alimentary tract and body walls.
There are two nephridia with ciliated tubes which open on the adoral side of the nerve ganglion, as in some Chaetopod Trochospheres (Harmer). Testes and ovaries are both present, but appear to ripen at different times, and the generative pore lies aborally behind the epistome. Segmentation is total, and there is an invaginate gastrula. The larva has a circular ciliated ridge, a sucker or depression, in the centre of the convex aboral surface surrounded by sense-cells, and on the same surface near the oesophagus a so-called 'dorsal organ' which is derived by invagination from the epiblast, and represents a supra-oesophageal ganglion, developing in Loxosoma fibres and cells. It is apparently connected with the sub-oesophageal ganglion of the adult which is formed from a solid mass of epiblast cells in the floor of the vestibule, but in Pedicellina at a late period. Eyes, or masses of pigment imbedding each a transparent lens, are found in the dorsal organ of most species of Loxosoma, together with a pair of ciliated pits, or in Pedicellina a single pit. The larval Loxosoma developes two buds, one on each side of but quite distinct from, the dorsal organ. The larva itself probably dies away (?). The adult gives origin to buds, one on each side of the body, which are set free.
The larval Pedicellina fixes itself by its oral extremity: the ciliated ring is retracted and degenerates, and the digestive tract undergoes a remarkable revolution in position. A part only of the original vestibular cavity persists, and the vestibular aperture of the adult is secondarily acquired. The aboral sucker and the 'dorsal organ' abort. The adult multiplies by gemmation from the creeping stolon. 'The Entoprocta, larval and adult, are true Trochospheres, possessing a ventral flexure of the alimentary canal, no true body cavity, and a pair of head kidneys.' 'The line between mouth and anus is ventral' (Harmer).
1To these add: Ascopodaria, a marine undescribed form dredged by the 'Challenger,' with an umbel of Pedicellina-like zooids borne upon a stalk (Lankester, 'Polyzoa,' Encyclopaedia Brit, xix); Urnatella gracilis, from the Schuykill river, U. S., a fresh-water form occurring in small colonies, consisting of a disc of attachment bearing one to six stems with urn-shaped joints and more or less branched, zooids like Pedicellina, dying at the end of the year, and stems persisting to bud the following year (Leidy, Journal Acad. Nat. Sc. Philadelphia (2), ix. pt. X. 1884, and Proc. of same, 1884). Barentsia, with a chitin-covered creeping stolon, giving off stems with lateral zooids, is supposed by Hincks (A. N. H. (5), vi. 1880) to belong here. Its full description by Vigelius (Bijdrag tot de Dierkunde, xi. 1884) has been inaccessible to me.
2Or it may be a portion of the ciliated ring of the larva. See Harmer, Q. J. M. xxvii. (2), 1886, p. 250.