MOLLUSCOIDA: POLYZOA. 



283 



avicularium is taken by a bristle, or seta, which is capable of 

 extensive movement. 



The endocyst is always soft, contractile, and membranous. 

 It lines the interior of the cells formed by the ectocyst, and is 

 reflected backwards at the mouth of the cell, so as to be inva- 

 ginated, or inverted into itself; and it finally terminates by 

 being attached to the base of the circlet of tentacles. This 

 invagination of the endocyst is more or less permanently pre- 

 sent in all the fresh-water Polyzoa. A portion of. the inner 

 surface of the endocyst, if not the whole, is furnished with 

 vibratile cilia. 



Fig. 102. — I. Fragment oi Flustra irjtticaia, one of the Sea-mats, natural size. 2. A 



single polypide of Valkeria, 'magnified, showing the orbicular crown of tentacles 



3. A polypide of Lophopus crysialUnus^ a fresh-water PplyzoSn, highly magnified, 



showing the horse-shoe-shaped crown of tentacles : a Tentacular crown ; b Gullet ; 



^ cStomach; rf Intestine ; <r Anus j '^Gizzard ; A Endocyst; ^Ectocyst; /Funiculus. 



The mouth of each polypide is surrounded by a crown of 

 tubular, non-retractile tentacles, which have their sides ciliated, 

 and are arranged sometimes in a circle and sometimes in a 

 crescent. In the fresh- water Polyzoa the tentacles are united 

 towards their bases by a funnel-shaped membrane, known as 

 the " calyx." The tentacles are borne upon a kind of disc, or 



avicularia, not as mere appendages or organs of any kind, but as peculiarly- 

 modified zooifls, having many singular points of affinity with the Brachio- 

 foda. The avicularia, like \\\e. fedicetlaria of the Echinodermata, continue 

 their movements long after the death of the animal. 



