VIII 



PHYLUM MOLLUSCOIDA 



323 



Structure of body-wall. Beneath the cuticle is an epidermis, 



consisting of a single layer of flattened polygonal cells, firmly 

 united together by their edges, so as to form a syncytium comparable 

 to that of the ectoderm of the Porifera. Beneath this there is 

 usually, but not always, a layer of muscle, which, when present, is 

 arranged in two strata an external composed of circular, and an 

 internal of longitudinal fibres. There is an extensive ccelome 

 lined in some forms by a definite ccelomic epithelium, in part 

 ciliated, while in others there is no such definite epithelium, but 



FIG. '258 bis. Cristatella mucedo. Entire eolony. (After Allman.) 



its place is taken by thin parietal and visceral layers of an 

 irregular cellular tissue the parenchyma. The body-cavities of 

 contiguous zooids are in some cases in free communication. Cross- 

 ing the coelome are strands, in some instances very numerous, of 

 spindle-shaped cells. In some cases two mesenteric bands suspend 

 the alimentary canal an anterior attached near the mouth and 

 a posterior passing from the ccecum to the aboral end of the 

 zocecium ; in most cases the latter, to which the special name 

 of fnniculns is given, is alone present. 



The alimentary canal has in all species the parts that have 



Y 2 



