CHAPTER XXI. 



THE BODY CAVITY, THE VASCULAR SYSTEM, AND THE 



VASCULAR GLANDS. 



The Body cavity. 



IN the Ccelenterata no body cavity as distinct from the 

 alimentary cavity is present ; but in the remaining Invertebrata 

 the body cavity may (i) 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 Verte- 

 brata and the Echinodermata ; the perihajmal 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 



