CHAPTER XVII. 



CHAETOGNATHA.* 



Hermaphrodite Coelomata, in which the body is divided into three 

 regions or segments. The head carries two groups of sickle-shaped 

 setae, and the central nervous system consists of cerebral and sub- 

 oesophageal ganglia connected by circumoesophageal commissures. 



The Chaetognatha are a very small group of transparent pelagic 

 organisms. Though small in numbers and presenting but little 

 variety of organisation, the group is one which, both in its adult 

 structure and embryonic development, is of the greatest interest 

 to naturalists. It is impossible to relate it to any of the great 

 phyla of the animal kingdoms, and, like so many of the groups 

 which we have recently considered, it must have assigned to it 

 the dignity of phyletic rank. 



In certain features of structure, e.g., in the form of the nervous 

 system, and in the relations and character of the coelom, it resembles 

 the Annelida, but it differs from them in the small number and in 

 the character of the segments of which the body is composed, in the 

 absence of distinct nephridia and a vascular system, and in the 

 structure of the embryo. For though the egg is small, and the whole 

 development takes place in the sea, there is no larval stage which 

 in any way recalls the trochosphere, and the coelom is developed 

 from archenteric diverticula, a feature which is found in no Annelid. 



As in the JBrachiopoda the coelom consists of three pairs of 

 chambers separated by transverse septa, and as in that group the 

 division of the coelom is effected subsequently to its establish- 

 ment as a single enteric diverticulum on each side. 



* A. Krohn, " Anat.-physiol. Beobachtungen ub. die Sagitta bipunctata," 

 Hamburg, 1844. R. Wilms, " De Sagitta mare germanicum circa insulam 

 Helgoland incolente" Beroliui, 1846. A. Kowalewski, "Embryologische Studien 

 au Wurmern u. Arthropoden," Mem. de I'Acad. Petersbourg, 16. 0. Hertwig, 

 " Die Chaetognathen, eine Monographic," Jena, 1880. B. Grassi, "I. Chetognathi," 

 Flora und Fauna d. Golfes von Neapel, 1883. S. Strodtmann, " Die Systematik 

 der Chaetognathen etc.," Arch. Naturg., 58, 1892. 



