380 POLYZOA [CH. XVI 



subdivisions are fresh- water. Fossil forms are numerous and the 

 Coralline Crag, a tertiary deposit, takes its name from the large 

 number of coral-like calcareous forms, sometimes described as 

 "corallines," which are found in it. 



There is a small class of animals regarded by many zoologists 

 as a division of Polyzoa, but which seems to others (with whom we 

 are inclined to agree) to be a completely independent group of 

 animals. These animals have no coelomic space, and both ends 

 of the alimentary canal are included in the ring of tentacles ; from 

 this circumstance they derive the name (lit. anus inside) ENTOPROCTA. 

 In this class the body consists of a stalk and a "cup." The edges 

 of the latter are fringed with a short row of ciliated tentacles 

 surrounding a disc on which both the mouth and the anus open. 

 When irritated these tentacles are bent inwards and the contraction 

 of a sphincter muscle causes the edge of the disc to be drawn over 

 them exactly as happens in a Sea-anemone. Sometimes in Pedi- 

 cellina the "cup" falls off and a new one is formed. Beneath the 

 disc is situated a nerve-ganglion and the genital organs are con- 

 tinuous with a short duct which opens in the centre of the disc. 

 The excretory system consists of a pair of true nephridia ending 

 internally in flame- cells, which open between the mouth and the 

 anus. 



All other Polyzoa are grouped together as ECTOPROCTA, and 

 these are subdivided into 



Order I. Gymnolaemata. 



With a few exceptions marine and having a circular 

 lophophore. Devoid of an epistome. 



Sub-order (1), Cheilostomata. An operculum covers the 

 orifice of the cell. Avicularia and vibracula often present. 

 Skeleton with more or less calcareous deposit in it. 



Sub-order (2), Cyclostomata. Cells tubular and ending 

 in circular mouths. No operculum. Calcareous skeleton. 



Sub-order (3), Ctenostomata. Body-wall soft. The 

 orifice of the cell is closed by the coming together of a mem- 

 brane, which is often beset with a fringe of spines. 



Order II. Phylactolaemata. 



Fresh-water forms with a horseshoe-shaped lophophore 

 (except JFredericella), an epistome, and statoblasts. 



