94 INTRODUCTION TO POLYZOA. 



fluid containing corpuscles, which, probably, 

 subserve excretion. In some instances part 

 of the cavity is ciliated. 



Body-wall structure. The structure 

 of the body-wall is as follows : The outer 

 layer is a horny cuticle (Ectocyst), and is 

 secreted by an inner layer of cells (Epidermis, 

 or Endocyst). Next come two layers of 

 muscular tissue (Parietal muscles), the first 

 arranged circularly and the other longitudinally. 

 Finally, there is lining the whole of the body 

 cavity, including the exterior of the alimentary 

 canal, an unequal layer of cellular tissue 

 (Coenosarc or Parenchyma) from which are 

 derived the parts which connect the various 

 members of the colony. In some of the 

 Polyzoa (Cyclostomata and Cheilostomata), 

 the external layer of the body-wall is 

 strengthened by a shelly deposit, and, in a few 

 instances, it is gelatinous. The obsolete word 

 cell is applied here to this protective outer 

 covering, as simpler than the accepted term 

 Zooecium. 



Alimentary canal structure. As 



regards the structure of the Alimentary canal, 

 it is lined internally by an epithelium of 



