CH^ETOGNATHA. 369 



(fig. 165, ge]. The former constitutes the ovary, and is situated 

 in front of the septum dividing 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.g) 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 Chaetopoda 1 . 



BIBLIOGRAPHY. 



(376) O. Biitschli. "Zur Entwicklungsgeschichte der Sagitta." Zeitschrift f. 

 wiss. Zoo!., Vol. xxni. 1873. 



(377) C. Gegenbaur. " Uber die Entwicklung der Sagitta." Abhand. d. na- 

 turforschenden Gesellschaft in Halle, 1857. 



(378) A. Kowalevsky. " Embryologische Studien an Wiirmern u. Arthropo- 

 den." Mem. Acad. Petersbourg, VII. ser., Tom. XVI., No. 12. 1871. 



MYZOSTOMEA. 



The development of these peculiar parasites on Crinoids has been 

 investigated by Metschnikoff (No. 380), Semper (No. 381), and Graff 

 (No. 379). 



The segmentation is unequal, and would appear to be followed by an 

 epibolic invagination. The outer layer of cells (epiblast) becomes covered 

 with cilia, and the inner is transformed into a non-cellular (?) central yolk 

 mass. At this stage the larva is hatched, and commences to lead a free 

 existence. In the next stage observed by Metschnikoff, the mouth, oeso- 

 phagus, stomach, and anus had become developed ; and two pairs of feet 

 were present. In both of these feet Chaetopod-like setae were present, which 

 in the hinder pair were simple fine bristles without a terminal hook. The 

 papilliform portion of the foot is at first undeveloped. The feet become 

 successively added, like Chaetopod segments, and the stomach does not 

 become dendriform till the whole complement of feet (5 pairs) are present. 



In the primitive covering of cilia, combined with a subsequent indication 



1 Langerhans has recently made some important investigations on the nervous 

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

 glia 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 embryolo- 

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



B. II. 24 



