CHAPTER XXI. 



THE BODY CAVITY, THE VASCULAR SYSTEM, AND 

 THE VASCULAR GLANDS. 



The body cavity. 



In the Coelenterata no body cavity as distinct from the alimentary 

 cavity is present ; but in the remaining Invertebrata the body cavity 

 may (1) take the form of a wide space separating the wall of the gut 

 from the body wall, or (2) may be present in a more or less reduced 

 form as a number of serous spaces, or (3) only be represented by 

 irregular channels between the muscular and connective-tissue cells 

 filling up the interior of the body. The body cavity, in whatever 

 form it presents itself, is probably filled with fluid, and the fluid in 

 it may contain special cellular elements. A well developed body 

 cavity may coexist with an independent system of serous spaces, as in 

 the Vertebrata and the Echinodermata; the perihsemal section of the 

 body cavity of the latter probably representing the system of serous 

 spaces. 



In several of the types with a well developed body cavity it has 

 been established that this cavity originates in the embryo from a 

 pair of alimentary diverticula, and the cavities resulting from the 

 formation of these diverticula may remain distinct, the adjacent walls 

 of the two cavities fusing to form a dorsal and a ventral mesentery. 



It is fairly certain that some groups, e.g. the Tracheata, with 

 imperfectly developed body cavities are descended from ancestors 

 which were provided with well developed body cavities, but how far 

 this is universally the case cannot as yet be definitely decided, and 

 for additional information on this subject the reader is referred to 

 pp. 294 — 297 and to the literature there referred to. 



In the Cheetopoda and the Tracheata the body cavity arises as a 

 series of paired compartments in the somites of mesoblast (fig. 350) 

 which have at first a very restricted extension on the ventral side of 

 the body, but eventually extend dorsalwards and ventralwards till 

 each cavity is a half circle investing the alimentary tract; on the 

 dorsal side the walls separating the two half cavities usually remain 



