CH^ETOGNATHA. 



305 



trunk division of the body cavity is marked in the adult by the 

 mesentery dividing into two laminae, which bend outwards to join the 

 body wall. 



The cephalic section of 

 the body cavity seems to 

 atrophy, and its walls to be- 

 come converted into the 

 complicated system of mus- 

 cles present in the head of 

 the adult Sagitta. 



In the presence of a sec- 

 tion of the body cavity in 

 the head the embryo of Sa- 

 gitta resembles Lumbricus, 

 Spiders, etc. 



The generative rudi- 

 ment of each side divides 

 into an anterior and a pos- 

 terior part (fig. 165, ge). 

 The former constitutes the 

 ovary, and is situated in 

 front of the septum divid- 

 ing the tail from the body ; 

 and the latter, in -the caudal region of the trunk, forms the testis. 



The nervous system originates from the epiblast. There is a 

 ventral thickening (fig. 165 B, v.y) in the anterior region of the trunk, 

 and a dorsal one in the head. The two are at first continuous, and 

 on becoming separated from the epiblast remain united by thin cords. 



The ventral ganglion is far more prominent during embryonic life 

 than in the adult. Its position and early prominence in the embryo 

 perhaps indicate that it is the homologue of the ventral cord of 

 ChaBtopoda 1 . 



BIBLIOGRAPHY. 



(376) 0. Biitschli. " Zur Entwickluugsgeschichte der Sagitta." Zeitschrift f. 

 wiss. Zool., Vol. xxin. 1873. 



(377) C. Gegenbaur. " Uber die Entwicklung der Sagitta." Abliand. d. natur- 

 forschenden GeseUschaft in Halle, 1857. 



(378) A. Kowalevsky. " Embryologische Stuclieu an Wiirmern u. Arthropoden." 

 Mem. Acad. Petersbourp, vii. ser., Tom. xvi., No. 12. 1871. 



FIG. 1C."). Two VIEWS OF A LATE EMBRYO OF 

 SAGITTA. A. from the dorsal surface. B. from 

 the side. (After Biitschli.) 



m. mouth; al. alimentary canal; r.g. ven- 

 tral ganglion (thickening of epiblast); t'p. epi- 

 blast; c.2)v. cephalic section of body cavity; so. 

 somatopleure ; s/j. splanchnopleure ; ge. gene- 

 rative organs. 



MYZOSTOMEA. 



The development of these peculiar parasites on Crinoids has been 

 investigated by Metsclmikoff (No. 380), Semper (No. 381), and Graft* 

 (No. 379). 



1 Langerhans has recently made some important investigations on the nervous 

 system of Sagitta, and identifies the ventral ganglion with the parieto-splanchnic ganglia 

 of Molluscs, while he has found a pair of new ganglia, the development of which is 

 unknown, which he calls the suboesophageal or pedal ganglia. The embryological 

 facts do not appear to be in favour of these interpretations. 



B. E. 



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