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. 



Chcetognatha. 



The discoveries of Kowalevsky (No. 378) confirmed by Biitschli 

 (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 great value for the more 

 general problems of embryology. The development commences after 

 the eggs are laid. The segmentation is uniform, and a blastosphere, 

 formed of a single layer of columnar cells, is the product of it. An in- 

 vagination takes place, the opening of which narrows to a blastopore 



FIG. 164. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and G 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 (m) has become continuous with 

 the alimentary tract, and the blastopore has become closed. 



m. mouth; al. alimentary canal ; ae. archenteron; M.p. blastopore; pv. perivisceral 

 cavity; sp. splanchnopleuric rnesoblast; so. somatopleuric mesoblast; (je. generative 

 organs. 



