6o8 THE INVERTEBRATA 



excreta in the absence of specific excretory organs. It can, however, 

 be hardly doubted that animals so small and with so great an area of 

 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 feature of this is the parietal system 

 of muscles which circle round the body wall. By their contraction 

 the internal pressure is raised and the polypide protruded. The re- 

 tractor muscle which runs from the lophophore to the opposite end of 

 the zooecium has an opposite action to the parietal system . The Polyzoa 

 are fascinating but exasperating objects under the microscope: they 

 emerge with infinite caution from the zooecium 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 coeno- 

 sarc. They are often incrusting like Membranipora and Flustra (hence 

 the name of '* sea mats "), with all their zooecia packed closely together 

 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. 417 A, 

 B). The parietal muscles, which in primitive polyzoa formed a con- 

 tinuous layer of circular muscles as in Chaetopoda, here form de- 

 tached groups running from the side walls through the coelom to the 

 frontal surface. When the muscles contract the latter is depressed 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. 417 C); to its 

 lower surface the parietal muscles are attached. When they contract 

 and the tentacles are extruded the sac fills with water, and when they 

 relax the sac empties. 



Polymorphism is a feature of polyzoan as it is of hydrozoan colonies. 

 Perhaps the most remarkable modifications are to be seen in the in- 

 dividuals known as vibracula and avicularia of such forms as Bugula 

 (Figs. 418 A, 419). The vibracula are nothing more than long bristles 



