ON THK DEVELOPMENT OF SAGITTA. 381 



but nevertheless differs from it in several important par- 

 ticulars. With regard to many of these, however, the 

 assumption of the adult form is very gradual, and takes 

 place rather by a process of differential growth than by any 

 real alteration of structure. Such changes as these are the 

 reduction in relative size of the ganglia, more especially of 

 the ventral ganglion, which in a Sagitta a week old is nearly 

 half the length of the animal, and also the formation of the 

 anterior pair of fins, which are absent in the larva. These 

 processes of development are comparatively unimportant, 

 and require no further description, but there are a few points 

 in which the young reared from the egg differ very materially 

 from the adult, and these must be followed further. They 

 concern chiefly the reproductive organs and their ducts, and 

 since these have been used as characters for determining the 

 systematic position of the group it is important that their 

 mode of origin should be accurately known. The oldest 

 young which I Avas able to rear still retained the four 

 primitive reproductive cells undivided, and none showed the 

 least trace of any reproductive duct either during life or in 

 section, and therefore to follow the development of these 

 organs it was necessary to use young specimens taken iu the 

 tow-net, in which Sagitta of almost all stages were usually 

 abundant. 



The youngest individuals which I obtained differed con- 

 siderably from those reared from the egg, and were obviously 

 considerably older ; they were fully twice as long, the anterior 

 paired fins were present, the ganglion much reduced, and the 

 parenchymatous tissue of the neck and behind the ganglion 

 had disappeared, and in every case the genital cells had 

 already divided to form groups of smaller cells. These 

 groups of cells occupy the positions of the primitive genital 

 cells, i. e. there is a pair of groups on each side, one in front 

 of, and the other behind the transverse septum ; in section 

 they appear lens-shaped, and lie closely pressed against the 

 body-wall on the level of the lateral fin. In each group all 

 the cells seem alike ; they are moderately large, with pro- 



