706 THE ANIMAL KINGDOM. 



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 J 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 aper- 

 ture 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). 



Ectoprccta. The zooids, or polypides, form a colony, or zoarium, 

 which is sometimes erect and either lamellate, or branching, and plant-like, 

 or else adherent to some foreign object, either living, e.g. crab, sea- weed, or 

 inanimate. The colony is fixed with rare exceptions. In Cristatella mucedo 

 it creeps about on a flat sole, and in the Selenaridae it is free and moves about 

 by the action of the vibracula. The polypide itself is covered by a cuticle or 

 ectocyst secreted by the ectoderm. This cuticle always remains thin and 

 flexible on the anterior part of the zooid which forms the so-called tentacle- 

 sheath, and can be invaginated and evaginated by the action of special 

 muscles (p. 236). The cuticle of the posterior part generally thickens, and 

 becomes resistent : it is gelatinous (Akyonidium ; Lophopus), or chitinoid, 

 and usually more or less opaque and sometimes partially, or wholly, calci- 

 fied. The calcareous matter in Lepralia, &c., is, however, said to be laid 

 down between two layers of ectoderm cells, which are covered externally 

 by a thin cuticle. The posterior region of the cuticle is known as zooecium, 

 and the collection of zooecia forming a colony constitute the coenoecium. 

 The zooids are often polymorphic. In both Ctenostomata and Cheilostomata 

 certain of them may be modified into stem-cells, and in the latter as Avi- 

 cularia, and Vibracula (p. 237). The Ooecia and Root-fibres are probably 

 to be regarded as organs (Vigelius). The ectocyst is lined by a soft layer 



