DEVELOPMENT AND HABITS 



189 



no internal opening and to act largely as a receptaculum 

 seminis. The oviducts open externally on the upper side at the 

 base of the lateral fin, close to the junction of the tail and the 

 trunk. 



The cavity of the tail is divided into two lateral chambers by 

 the extension backward of the median vertical mesentery. In 

 each of these a testis and a vas deferens are found. The testes 

 are solid ridges formed by the growth of the 

 lining cells of this part of the body-cavity ; 

 the cells mature into spermatozoa, which 

 break off and float freely in the coelomic fluid. 

 At the breeding season the whole tail may 

 be crowded with masses of spermatozoa, which 

 are kept in a more or less regular circulation 

 by the ciliated cells lining the body-wall. 

 The vas deferens opens internally into the 

 space where the spermatozoa lie, and at the 

 other end into a vesicula seminis, which 

 opens to the exterior. The position of the 

 latter structure varies, and is of some syste- 

 matic value. 



The eggs are laid in the water and as a 

 rule float at the surface of the sea. Spadella 

 cephaloj)tera is, however, an exception to this 

 rule, as it attaches its eggs by means of a 

 gelatinous stalk to sea-weeds. The segmen- 

 tation of the ovum is regular, and gives rise 

 to a two-layered stage or gastrula, which 

 opens by a pore, the blastopore. This does 

 not, however, become the mouth, but closes FlG io3_^sw e tt 

 up and the mouth arises at the opposite loptera. 

 pole. Perhaps the most interesting feature 

 of the development of Sagitta is that the 

 cells destined to form the reproductive 

 organs separate from the other cells of the 

 embryo at a very early date, whilst it is 

 still in the gastrula stage. There is no 

 larval form, but the young hatch out from the egg in a state 

 resembling the adult in all respects but that of size. 



Habits. The Chaetognatha are essentially pelagic, and re- 



Dorsal view. 

 x30. (FromHertwig.) 



a, Cephalic ganglion ; 



b, commissure to ventral 

 ganglion ; c, olfactory 

 organ ; d, alimentary 

 canal ; e, ovary ; /, ovi- 

 duct ; (j, testis ; h, vesi- 

 cula semiualis. 



