CHAPTER XV. 



CH/ETOGNATHA, MYZOSTOMEA AND GASTROTRICHA. 



THE present chapter deals with three small isolated groups, 

 which only resemble each other in that the systematic position of 

 all of them is equally obscure. 



Chcstognatha. 



The discoveries of Kowalevsky (No. 378) confirmed by 

 Btitschli (No. 376) with reference to the development of Sagitta, 

 though they have not brought us nearer to a knowledge of the 

 systematic position of this remarkable form, are nevertheless of 



FIG. 164. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after 

 Biitschli and B after Kowalevsky.) The three embryos are represented in the same 

 positions. 



A. The gastrula stage. 



B. A succeeding stage in which the primitive archenteron is commencing to be 

 divided into three parts, the two lateral of which are destined to form the body 

 cavity. 



C. A later stage in which the mouth involution (iii) has become continuous with 

 the alimentary tract, and the blastopore has become closed. 



in. mouth; al. alimentary canal ; ac. archenteron ; bl.p. blastopore ; pv. perivisceral 

 cavity; J/. splanchnopleuric mesoblast; so. somatopleuric mesoblast ; ge. generative 

 organs. 



