532 THE INVERTEBRATA 



epithelium in contact with the water are able to rid themselves of 

 excreta in a simpler fashion. 



As triploblastic metazoa with a centralized nervous system the 

 Polyzoa possess a more efficient contractile mechanism than the 

 hydroids. The most prominent features of this are the parietal 

 system of muscles which circle round the body wall. By their con- 

 traction the internal pressure is raised and the lophophore extended. 

 The retractor muscle which runs from the lophophore to the opposite 

 end of the cell has an opposite action to the parietal system. The 

 Polyzoa are fascinating but exasperating objects under the micro- 

 scope : they emerge with infinite caution from the cell and withdraw 

 with incredible rapidity. With the lophophore a flexible part of the 

 body wall is also invaginated and this is called the tentacle sheath. 



The colonies of polyzoa differ greatly from those of hydrozoa in 

 their habit and this is largely due to the absence of a connecting 

 coenosarc. They are often incrusting like Membranipora and Flustra 

 (hence the name of *' sea mats "), with all their cells packed closely to- 

 gether in a single layer ; they may also be slender or massive ; in the 

 latter case they have a superficial resemblance to the actinozoan 

 corals. While the outer layer of the body wall is often horny or 

 flexible it frequently becomes incrusted with calcium carbonate and 

 thus rendered rigid. 



In the incrusting Polyzoa, especially the Cheilostomata, the 

 zooecia are rigid boxes, in contact with one another along all four sides 

 and with the substratum at the bottom. These are usually strongly 

 calcified and only the top of the box, the frontal surface, is flexible 

 (Fig. 392 A, B). The parietal muscles, which in primitive polyzoa 

 formed a continuous layer of circular muscles as in Chaetopoda, here 

 form detached groups running from the side walls through the coelom 

 to the frontal surface. When the muscles contract the latter is de- 

 pressed and the lophophore is protruded. The process of calcification 

 may extend to the frontal membrane and the mechanism of protrusion 

 has then to be altered. In one large group of the Cheilostomata, there 

 is a membranous diverticulum of the ectoderm under the calcareous 

 frontal surface. This is called the compensation sac (Fig. 392 C); to 

 its lower surface the parietal muscles are attached. When they con- 

 tract and the tentacles are extruded the sac fills with water, and 

 when they relax the sac empties. 



Most of the Polyzoa are marine and' are amongst the most familiar 

 objects of the beach. A complete division, the Phylactolaemata, are 

 freshwater and it is one of these which is figured here (Fig. 391). The 

 marine forms possess a variety of free-swimming larvae, which are of 

 the trochosphere type. In the Phylactolaemata, certain internal buds 

 called statoblasts are formed from lens-shaped masses of cells on the 



