358 



KEMATHELMIXTHES. 



the region of the mouth two lateral groups of hooks which function 

 as jaws. 



The nervous system consists, according 

 to Krohn, of a cerebral ganglion on 

 which the eyes are situated, and a ven- 

 tral ganglion placed in about the middle 

 of the body length. There are in addition 

 two ganglia near the mouth, which may 

 be considered as the subcesophageal gan- 

 glia, and are connected with each other 

 and with the cephalic ganglion by oeso- 

 commissures. 



l^. 288.Sagitta (Spadella) 

 cephalopttra, magnified 30 

 times, viewed from the dorsal 

 side (after O. Hertwig). F, 

 posterior fin; G, supra- 

 O3sophageal ganglion ; Te, ten- 

 tacles ; R, olfactory organs ; 

 Ov, ovary; Od, oviduct; T, 

 testis; Vd, vas deferens; Sb, 

 vesicula seminalis. 



[The common view now is that the large 

 ventral ganglion of the middle of the body, 

 which is connected with the cerebral by com- 

 missures, is homologous with the suboesophageal 

 ganglia of other types.] 



The straight alimentary canal is at- 

 tached to the body wall by a dorsal and 

 ventral mesentery from the oesophagus 

 backwards, and opens to the exterior at 

 the base of the long tail, which terminates 

 in a horizontal fin (fig. 288). 



[The body cavity is well developed, and 

 divided by the dorsal and ventral mesenteries into 

 two parts, and again by two transverse verti- 

 cal septa into a cephalic section, a section in 

 the body, and finally a caudal section. Vas- 

 cular and excretory organs are absent.] 



Reproduction. The Chcetognatka are 

 hermaphrodite, and possess paired ovaries, 

 which open by two apertures at the base 

 of the tail and are connected with 

 seminal pouches. The testes also are 

 paired, and situated posteriorly to the 

 ovaries in the tail; their products pass to 

 the exterior by openings at the sides of 

 the tail. Segmentation is complete, and 

 leads to the formation of a blastosphere. 

 One side of this becomes invaginated so 

 that the segmentation cavity is obliterated 

 and a gastrula is formed, in the entoderm 



