42 PRIMITIVE ANIMALS 



consists of a number of vascular spaces containing 

 blood. The heart is perforated by ostia as in an 

 ordinary Arthropod, and the reproductive organs are 

 built on the complicated Arthropodan plan. But in 

 every segment of the body, except the few extreme 

 anterior and posterior segments, there is present a 

 pair of exeretory organs, just as in a typical 

 Annelid. These excretory organs are small coiled 

 tubes which open by ciliated funnels internally into 

 small dilated end-sacs, and they open to the exterior 

 at the bases of the legs. 



We must, however, trace the development of 

 these organs in ordei; to appreciate their full signi- 

 ficance for the solution of our problem (see Fig. 7). 



Peripatus develops directly from the egg without 

 passing through any larval stage, and in a very young 

 embryo soon after the three primary layers of cells 

 have been formed, viz. the ectoderm, endoderm and 

 mesoderm, we find that the mesoderm acquires a 

 cavity, a true coelom, one wall of which is applied 

 to the ectoderm and the other to the endoderm 

 (Fig. 7 A). The mesoderm, further, becomes seg- 

 mented, so that its cavity, the coelom, is cut up 

 into a number of partitions separated from one 

 another by transverse septa, and in this stage it 

 exactly resembles a young worm. The subsequent 

 fate of these coelomic partitions is of fundamental 

 importance. Each coelomic partition divides into 



