THE DEVELOPMENT OF THE POLYCH^TA. 213 



being merely developed from some part of the walls of the 

 perivisceral cavity, in which they eventually freely float, mak- 

 ing their way out in a manner which is not quite understood 

 at present ; probably, however, through temporary or perma- 

 nent apertures at the bases of the parapodia. In many, the 

 segmental organs appear to serve as excretory ducts. As a 

 rule, the polychaetous Annelids are dioecious; but some ( 

 Protula, Fig. 54) are hermaphrodite. The ova undergo their 

 development within the body of the parent in some species 

 of Eunice ; in pouches attached to the body in Exogone ; in 

 masses of gelatinous matter which adhere to the tubes of the 

 vermidom in Protula ; beneath the elytra in Polynde cir- 

 rata ; in the cavity of the opercular tentacle in some Spir- 

 orbes ; while, in other cases, they appear to become, almost 

 immediately, free ciliated embryos. 



The vitellus undergoes division, and is converted, as in 

 the case of the Oligochceta and Hirudinea, into blastomeres 

 of two kinds. This contrast between the two components of 

 the embryo commences with the division of the vitellus into 

 two, inasmuch as the first fissure is usually so directed as to 

 divide the yelk into unequal portions. Both subdivide, but 

 the smaller much faster than the larger ; so that the former 

 becomes converted into very small blastomeres, which grad- 

 ually envelop the larger blastomeres resulting from the sub- 

 division of the latter. The larger included blastomeres are 

 destined to form the alimentary tract; the smaller peripheral 

 ones, on the other hand, give rise to the ectoderm, and to the 

 nervous ganglia. 1 As in the Ottgochasta and Hirudinea, 

 again, the mesoblast forms a thick band on each side of the 

 median ventral line, and its transverse division originates the 

 segmentation of the body. But, generally, the development 

 of the protosomites, as these segments might be called, does 

 not occur until some time after the embryo has been hatched. 

 The somites increase in number by the addition of new ones 

 between the last and the penultimate somite. 



The embryos of the Polychcet* differ from those of the Oli- 

 gochceta and Hirudinea in being ciliated. In some cases, the 

 cilia form a broad zone which encircles the body, leaving at 

 each end an area, which is either devoid of cilia, or, as is fre- 

 quently the case, has a tuft of long cilia at the cephalic end. 

 Such larvae are termed Atrocha. 



In other embryos the cilia are arranged in one or more 



1 Claparede and metschnikoff, " Beitriige zur Kenntnias der Entwickelungs- 

 geschiohte der Chaetopoden," 1868. 



