MOLLUSCA: POLYZOA. 375 



processes," differ a good deal in shape, but consist essentially 

 of " a movable mandible and a cup furnished with a horny 

 beak, with which the point of the mandible is capable of being 

 brought into apposition " (Busk). In shape the avicularia 

 often closely resemble the head of a bird, and they are in 

 many respects comparable with the " pedicellarise " of the 

 Echinodermata, keeping up a constant snapping movement, 

 which continues long after the death of the general colony. 

 In the "vibracula," the place of the mandible of the avicu- 

 larium is taken by a bristle, or seta, which is capable of exten- 

 sive movement. In many cases the cells are also furnished 

 with globular sacs or pouches (" ovicells " or "oocysts"), ap- 

 pended to them, and serving as marsupial pouches for the ova. 

 Ovicells are only known in the marine Polyzoa, and the ova 

 are liberated by their ultimate rupture. 



It is generally believed that the avicularia, vibracula, and 

 ovicells are really undeveloped polypides or modified zooids. 

 Good authorities also believe that the "cells" or "zooecia" 

 themselves are not to be regarded as mere skeletal structures, 

 but that they have a life independent of that of their con- 

 tained polypides, and that they can continue to live and pro- 

 duce new polypides after the death of the latter. They are 

 regarded, in fact, as separate zooids. 



The endocyst is always soft, contractile, and membranous ; 

 and, according to Sars, is wanting in Rhabdopleura. 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 invaginated, 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 present in all the 

 fresh-water Polyzoa. The epithelium lining the inner surface 

 of the endocyst is furnished with vibratile cilia. 



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 on a kind of disc, or 

 stage, which is termed by Professor Allman the "lophophore." 

 In the majority of Polyzoa including almost all the marine 

 species the lophophore is circular (fig. 199, 2) ; but in most 

 of the fresh-water forms it has its neural side extended into 

 two long arms, so that the entire lophophore becomes crescen- 

 tic or "horse-shoe-shaped" (fig. 199, 3); hence this section is 

 sometimes collectively termed the " Hippocrepian " Polyzoa. 



