POLYZOA. 707 



or endocyst formed apparently in marine Polyzoa by the fusion of two 

 layers of cells. It has recently been stated, however, that an ectoderm 

 layer may always be distinguished by the use of silver nitrate. In the 

 P hylactolaemata a layer of ciliated epithelium lines the coelome, and dis- 

 tinct muscular layers, external circular, and internal longitudinal, intervene 

 between it and the ectoderm, at least in Alcyonella fungosa. There is a 

 common coelome for all the zooids in the colony of the P hylactolaemata, 

 but in the Gymnolaemata the coelome of every zooid is isolated by septa. 

 In these septa and in the walls of the zooecia, where they touch one another 

 there are in some forms, groups of perforations, or communication-plates, 

 which are covered by a columnar epithelium and are connected by the 

 funiculi (p. 235). 



The mouth and anus are approximated, and the line between them, 

 perhaps, marks a mid-ventral surface. The ganglion is situated between 

 them. It is minute and double in P hylactolaemata where it has a 

 narrow commissure encircling the oesophagus, and supplies nerves to the 

 lophophore, tentacles (?) and digestive tract. Sensory cells resembling 

 those of the Entoprocta have been detected on the tentacles of Alcyonella 

 fungosa, &c. The mouth in the P hylactolaemata is overhung by a small 

 ciliated mobile lobe, the epistome, perhaps homologous with the same 

 structure in Entoprocta. It lies in the centre of a disc, or lophophore, 

 either circular (Gymnolaemata}, or horse-shoe shaped, i.e. crescentic (P hylac- 

 tolaemata}, along the edges of which are ranged in single series a row of 

 tentacles which are postoral in position. They are hollow, and their 

 cavities are extensions of the coelome : they are covered on the aboral and 

 adoral surfaces by ciliated epithelium, on the action of which depends the 

 food supply. Their number varies much ; they are flexible and move 

 swiftly bending to one or the other side, or coiling and uncoiling together 

 in a spiral. Muscular bands have been detected inside them on the aboral 

 and adoral surfaces. In the P hylactolaemata the bases of the tentacles are 

 protected externally by a thin membrane or calyx. The alimentary canal 

 retains a U-shaped curvature. It consists of a pharynx, followed in many 

 instances by a ciliated oesophageal region, separated by a slight constric- 

 tion from a stomach, the cells lining which contain a brown pigment. 

 There is interposed between the stomach and the oesophagus in Ctenosto* 

 mata a gizzard with thickened muscular walls, and containing gastric teeth 

 or pointed processes. The stomach is tied to the endocyst by one or 

 more funiculi, and is often V-shaped, as in Membranipora (Fig. 10 A, p. 

 235). The bent up pyloric portion is always ciliated in Gymnolaemata. 

 The length of the intestine varies. The coelome is roomy, and contains 

 a liquid in which float corpuscles derived, it is said, from the funicular 

 tissue. 



The P hylactolaemata are hermaphrodite, as are some Gymnolaemata ; 



z z 2 



