82 PRIMITIVE ANIMALS 



Appendiculata, the body muscles of the Vertebrate 

 are also metamerically segmented, being derived in 

 the embryo from the walls of the coelom which in 

 the Vertebrate, as in the Earthworm, is cut up into 

 repeated, metamerically segmented, blocks. The only 

 difference between the metameric segmentation of 

 the Vertebrate and of the Annelid is that in the 

 former only the dorsally situated parts of the coelom, 

 the myotomes, are involved in the segmentation, the 

 perivisceral coelomic cavity being unsegmented, 

 whereas in the latter the coelom is completely 

 divided, both dorsally and ventrally, by a series of 

 septa which partition the perivisceral coelom into 

 metamerically repeated compartments. This is, how- 

 ever, a small difference and need not obscure the 

 central fact that Annelids and Vertebrates, including 

 Amphioxm, resemble one another fundamentally in 

 the type of their metameric segmentation. 



There is still another feature in which Amphioxus 

 betrays a similarity to the Annelid, namely in its 

 excretory organs (Fig. 18 nph.\ These organs in 

 Amphioxus are a series of metamerically repeated 

 tubes which end in closed excretory cells, known as 

 solenocytes, bearing an extraordinarily accurate re- 

 semblance to the excretory cells which terminate the 

 segmental nephridia of many marine Annelids. There 

 can hardly be any doubt that these excretory tubes 

 in Amphioxus are true nephridia of ectodermal origin, 



