384 CHAETOGNATHA [CH. XVII 



cavities the cells lining the body-wall are heaped up and form a 

 testis (j, Fig. 180). The cells divide up into spermatozoa, which 

 float in the coelomic fluid and are kept in motion by some of the 

 ciliated cells lining the body-cavity. The spermatozoa escape 

 through a pair of short vasa deferentia which open on the one 

 hand into the coelom and on the other to the exterior on the 

 tail. Each has on its course a well-marked vesicula seminalis 

 (Fig. 180). 



The ova are transparent and pelagic. The cells destined to 

 form the reproductive organs of the adult are early set apart and 

 distinguishable. The development is entirely embryonic, no larval 

 form being recognisable. 



The Chaetognatha consist of three genera, Sagitta, Spadella 

 and Krohnia, amongst which some twenty-three species are divided. 

 The genera differ from one another chiefly in the arrangement of 

 the armature on the head and in the disposition of the fins, which 

 are always horizontally placed and supported by fine skeletal rods. 

 A caudal fin exists in addition to one (Spadella and Krohnia) or 

 two (Sagitta) pairs of lateral fins. 



The Chaetognatha are with hardly an exception pelagic, that 

 is they live near the surface of the sea, and as is usually the case 

 with animals which frequent the surface of the ocean they are 

 transparent. At certain times of the year they are found in 

 incredible numbers swimming on the surface by the muscular con- 

 traction of their bodies, their fins acting as balancers and having no 

 movement of their own. At other seasons they descend and are 

 taken at depths varying from 100 to over 1000 fathoms. The cause 

 of their descent is unknown. Their food consists of Infusoria, small 

 larvae and small Crustacea. 



The zoological position of the Chaetognatha is obscure. They 

 show no relationship with any of the larger groups ; possibly their 

 nearest existing allies are to be found amongst certain aberrant 

 Nematoda, such as Chaetosoma, but at present too little is known 

 to make any close comparison possible. 



