610 INVERTEBRATE MORPHOLOGY. 



time these folds are gradually constricted off from the intes- 

 tine, and at the same time are divided transversely into a 

 number of sacs lying one behind the other, their number in- 

 creasing as the folds are separated from before backwards 

 from the intestine, until in A. lanceolatus there may be as 

 many as sixty-one. These sacs are the primitive mesodermic 

 somites, and the cavities they contain are the primitive cce- 

 lornic cavities. At first entirely dorsal in position, the various 

 sacs later on extend ventrally, those of opposite sides meeting 

 below the intestine ; and still later the cavities of these ven- 

 tral extensions fuse to form a continuous ccelom extending the 

 entire length of the body on the ventral surface, and forming 

 what is termed the splanchnoccel. This becomes eventually 

 separated by a layer of connective tissue from the more dor- 

 sal portions of the somites, which remain distinct from each 

 other throughout life and are termed the myoccels. The future 

 history of the t\vo portions of the mesoderm thus formed 

 is very different. The walls of the splauchuocoel remain 

 thin, and the cavity well marked (Fig. 279, co), but in the 

 myoccels the cells forming the median walls become converted 

 into longitudinal muscle-fibres (ni) which traverse the entire 

 length of each myoccel, filling it almost completely, and are 

 inserted into plates of connective tissue which develop be- 

 tween the various myoccels and separate them from one an- 

 other. At the same time each myoccel becomes bent, so that 

 its dorsal portion is directed downwards and forwards and 

 its ventral portion downwards and backwards, each muscle- 

 plate having in a longitudinal section of the body a <-shaped 

 appearance and fitting into the one in front of it. When 

 the epipleural folds develop, both the splauchnoccel and 

 the muscle-plates are continued into them, the muscle-plates 

 lying to the outer side of the splanchnoccel, and their fibres 

 here having for the most part a transverse direction, in- 

 stead of a longitudinal one, as in their upper portions. 

 Owing to the myoccels being practically obliterated by the 

 muscle-plates, the ccelorn of the adult is principally formed 

 of the splanchuoccel, but other spaces also occur which are 

 probably schizoccelic in origin and form various lacunae 

 throughout the body. 



