472 



ZOOLOGY 



SECT. 



but the stage intervening between the completion of the gastrula 

 and the commencement of the segmentation of the mesoderm 

 bands corresponds to the trochophore in essential respects ; and in 

 some forms there is recognisable a feebly developed circlet of cilia 

 comparable to the prototroch, and in some a pair of head-nephridia. 

 Impregnation and the development of the embryo take place 



externally in all the Chaetopoda, 

 with a very few exceptions in which 

 development occurs in the ccelome 

 or in the interior of a dilated seg- 

 mental organ. In the Polychseta, in 

 the great majority of cases, fertilisa- 

 tion takes place by the sperms 

 coming in contact with the ova 

 when both have become discharged, 

 and the development of the embryos 

 enw t goes on while they are floating freely 

 in the sea. There are a few cases 

 in which the impregnated ova are 

 received into a sort of brood-pouch 

 and there pass through at least the 

 earlier stages of their development. 

 Such a brood-pouch is formed in 

 certain Phanerocephala by the 

 raising up of the integument on the 

 ventral surface. In some species 

 of Polynoe and allied genera, the 

 fertilised ova and the resulting 

 embryos adhere in masses to the 

 dorsal surface under the shelter of 

 the elytra ; in some other Polychseta 

 (certain Syllidea) they are stuck by 

 means of a viscid secretion to the 

 dorsal or the ventral surface (Fig. 

 385), or to the cirri. In certain 

 Cryptocephala (Fig. 386) they de- 

 velop in a cavity in the operculum ; 

 in others, in the interior of the 

 tube, between the body of the worm 

 and the inner surface of the latter, 

 or on its outer surface. In some, 

 again, though the ova do not remain in any way attached to the 

 parent worm, they may be deposited in clumps or packets enclosed 

 in gelatinous matter. Usually they have no other covering but the 

 egg-membrane. 



The segmentation of the ovum in the Polychseta is unequal. 

 In the great majority the inequality between the megameres and 



FIG. 385. Fionosyllis elegans. 



Dorsal view of female with advanced 

 embryos attached to the ventral sur- 

 face, d. c. dorsal cirri ; emb. embryos. 

 (From Potts, after Pierantoni.) 



