

PHYLUM ANNULATA 443 



Oligochaeta, but the stage intervening between the completion of 

 the gastrula and the commencement of the segmentation of the 

 mesoderm bands corresponds to the Trochosphere in essential 

 respects, and in some forms there is recognisable a feebly de- 

 veloped 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 Chsetopoda, with a very few exceptions, in 

 which development takes place in the ccelome or in the interior 

 of a dilated segmental organ. In the Polychseta, in the great 

 majority of cases, fertilisation takes place by the sperms coming 

 in contact with the ova when both have become discharged, 

 and the development of the embryos 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 Errantia 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 Errantia they are stuck by means 

 of some viscid secretion all over the dorsal surface, or they may 

 adhere singly to the ventral cirri. In certain Tubicola (Fig. 346) 

 they develop 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 

 micromeres is very marked. In some Serpulids, however, the differ- 

 ence is very slight, and the. two sets of cells are at first scarcely 

 distinguishable. In such cases the cells arrange themselves in 

 such a way as to form the wall of a hollow sphere, the Nastula, 

 with an internal closed cavity, the segmentation cavity. The 

 megameres, which may or may not have been distinct from 

 the first, lie on one side of the blastula ; and soon this side 

 becomes invaginated (Fig. 347, A), the result being the forma- 

 tion of an emlolic gastrula. In the great majority of forms, 

 however, an epibolic gastrula is formed after the manner already 

 described in the case of Nereis; but forms of the process 

 of gastrulation intermediate between these two extremes have 

 been observed to occur. The blastopore of the gastrula, however 

 formed, does not give rise directly either to the mouth or to the 

 anus. It becomes elongated into a slit which becomes closed up, 





