POLYZOA. 



237 



As we have just said, most of the Polyzoa form colonies, the size of which is 

 increased by budding, exactly as with the sertularians. In form they vary greatly ; 

 some, as Gemellaria, forming branching tree-like colonies, some, like Membranipora, 

 spreading in flat sheets over the surface of submarine objects, while others, of which 

 we may mention Alcyonidium, form soft and moss-like sheaths upon the rock-weed 

 between tide marks. In Rhabdopleitra, Laguncula, etc., a creeping root-stalk is 

 formed, from which arise the cells, in a manner which strikingly resembles that of 



some of the campanularian hydroids ; 

 while in Loxosoma the individuals 

 are separate, and no colonies occur. 



The chitinous or calcareous skele- 

 ton is composed of a series of cups, 

 each of which contains one of the 

 polypides, or individuals of the colony. 

 Each polypide is fastened to the interior of 

 its cell, but the mode of attachment is such 

 that it can, at will, partially extend itself or, at 

 the approach of danger, it can withdraw all its soft 

 and delicate organs. To better afford protection from 

 external harm, each cell of the colony is frequently 

 armed with strong teeth or long spines, or there may even 

 be an operculum developed, a little lid, which, when the ani- 

 mal is retracted, closes the opening through which the body 

 extends itself at other times. 



When the polypide is extended, the most prominent fea- 

 ture is a disc, known as the lophophore, from which arises a 

 more or less circular row of tentacles. Each of these ten- 

 tacles is ciliated, and the constant motion of these small 

 organs produces in the surrounding water currents which 

 flow to the mouth, which in some is situated within, in others 

 without, the circle of tentacles. The mouth communicates 

 with a large pharynx, which in turn empties into the oesopha- 

 gus, the distinction between these two being frequently em- 

 phasized by the presence of a valve. In several forms the 

 oesophagus terminates posteriorly in a muscular gizzai-d, the 

 function of which is to thoroughly triturate the food before 

 it enters the stomach, the next division of the alimentary 

 tract. The stomach is lined with small follicles, which are 

 Fig. 232.- Anatomy of Paiudv- regarded as hepatic in function, while its upper portion bears 

 j!»sotCgus?c:an™'r^!ttom: numerous cilia, which, by their constant motion, keep the 

 m^L™err'ruet^e'or|Tnsf'*' food in a State of agitation. The stomach is flexed upon 

 itself, and after the food is digested, the excrementa are 

 passed to the intestine, and thence out at the vent, which is placed close to the 

 mouth. 



No heart or circulatory organs exists in the Polyzoa, but the products of digestion 

 pass through the walls of the stomach into the body cavity, where they bathe the 

 various poitions of the body. The nervous system is chiefly composed of a central 

 ganglion placed between the mouth and the anus. In some forms, nervous cords-have 



