237 



As we have just s:iiil, most (if tlie Polyzoa form colonies, the size of wliidi is 

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

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

 sjireading 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 j-ock-weed 

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

 formed, from which arise the cells, in a manner which strikingly i-esembles that of 



some of the campauularian 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 

 olypides, or individuals of the colony, 

 jiolypide is fastened to the interior of 

 but the mode of attachment is such 

 an, at will, partially extend itself or, at 

 h of danger, it can withdraw all its soft 

 organs. To better afford protection from 

 xternal 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 o])ening through wliich the body 

 extends itself at other times. 



When the polypide is extended, the most prominent fea- 

 ture is a disc, known as the loj)lio])hore, from wliicli arises a 

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

 tacles is ciliated, an<l the constant motion of these small 

 organs produces in the surrounding water currents wliich 

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

 without, the circle of tentacles. The mouth communicates 

 with a large jjharynx, which in turn cm])ties into the ojsopha- 

 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 thorougldy ti'iturate the food before 

 it enters the stomach, the next division of the alimentary 

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

 regarded as hepatic in function, while its upper jiortion bears 

 numerous cilia, which, by their constant motion, keep the 

 food in a state of agitation. The stomach is flexed u])on 

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

 and thence out at the vent, which is jilaced close to the 

 mouth. 



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

 ]i.ass through the walls of the stomach into the body cavity, where they bathe the 

 \ arious portions of the body. The nervous system is cliiefly composed of a central 

 ganglion ])laced between tin- mouth and the anus. In some forms, nervous cords have 



