368 HISTORY OF THE HUMAN BODY 



have two avenues of escape, the nephridia and the original 

 openings of the gonadic sacs, and while so far as is known 

 no animal exists that utilizes both methods, either one may 

 become specialized to subserve this function. Furthermore, 

 in an animal with many somites it is not necessary that ovaries 

 or testes should develop in each pair of metaccelic sacs, but 

 these may be confined to a few pairs or even a single pair, 

 in which either the nephridia or the gonadic openings develop 

 into special excurrent ducts for the liberation of the germ 

 cells. 



The conditions of this second hypothetical historic stage' 

 are realized in almost every detail by the annelid worms, 

 allowing for a few modifications. [Cf. Fig. 139.] To some 

 this indicates the conclusion arrived at independently by the 

 consideration of other systems, that these animals lie very 

 near the main stem of vertebrate ancestry, but to others this 

 is no more than a case of parallel development, in no way 

 more remarkable than countless other adaptive resemblances, 

 such as the instance of the eye in cephalopods and vertebrates. 

 However this may be, the example of the annelid is most use- 

 ful in showing us that animals can exist in precisely the con- 

 dition of - the hypothetical form indicated by the study of 

 vertebrate embryology and constructed from the data thus 

 furnished. 



From this second stage, which must be very near the actual 

 condition in the ancestor of modern vertebrates, the final 

 type may be reached by the introduction of a few slight and 

 very natural modifications. The first of these concerns the 

 metaccelic sacs and consists, first, of the breaking down of the 

 dissepiments between the body segments, thus throwing all 

 the sacs of each side into one, and secondly, a similar loss, at 

 least in part, of the ventral mesentery, making the two lateral 

 sacs confluent below the intestine and allowing this latter to 

 swing free in the cavity, suspended dorsally. 



Thus, for the first time is reached a single secondary body 

 cavity or metaccele (the definite (( ccelom"), lined ivith peri- 

 toneum, which is re-fleeted along the mid-dorsal line and 



