CHAPTER XII. 



CH^TOGNATHA. 



The Chaetognatha occupy an altogether isolated position as 

 regards their structure and mode of development. Though 

 they most nearly resemble the Annelida in peculiarities of 

 structure, they differ from this group in important embryo- 

 logical features. Among the most characteristic peculiarities 

 of the development of the Chaetognatha are to be mentioned 

 the origin of the mesoderm by the formation of two archen- 

 teric diverticula and the early differentiation of the funda- 

 ment of the sexual organs. Owing to the absence of peculiar 

 larval forms, it is evident that the development of the 

 Chgetognatha is abbreviated. The developmental history of 

 the Chaetognatha has been made knovrn chiefly by Gegen- 



BAUR, KOWALEVSKY, BtJTSCHLI, and 0. HeRTWIG. 



The eggs of the Chgetognatha (Sagitta) after fertilization 

 has taken place are discharged into the surrounding water. ^ 

 They are spherical, transparent, and contain numerous clear 

 yolk spherules. They are surrounded by a vitelline mem- 

 brane and an outer gelatinous mantle. Cleavage must be 

 considered as total and equal, and leads to the formation of 

 a regular blastula, which is characterized by the tall pris- 

 matic form of its cells, which are grouped about a compara- 

 tively small cleavage cavity. One half of the embryo soon 

 flattens and invaginates, whereby the cleavage cavity is 

 reduced to a fissure. In this way a very regular invagina- 

 tion gastrula is formed (Fig. 161 A), the blastopore of 



^ [BovERi states that the eggs at the time of ovipositing in passing 

 through the narrow orifices assume an elongated form, but that they 

 recover the rounded shape in the water. When the eggs are laid the first 

 polar spindle is already formed, and every egg also contains a spermato*- 

 zoon. — K.] 



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