IV 



VERMES— NEPHRIDIA 



237 



open communication between the body cavity and the exterior, and 

 serve chiefly for conducting the waste products of metabolism out 

 of the body. Since the genital products are, in many Annulata, 

 developed out of the endothelium of the body cavity, then free them- 

 selves from the matrix, and ripen when floating freely in the coelomic 

 fluid, an opportunity is given to them also of reaching the exterior 

 through the nephridia. The nephridia thus frequently undertake, in 

 addition to their purely excretory function, the transmission of the 

 genital products to the exterior. This secondary function may often 

 become the principal function in some of the nephridia, which 

 may then undergo a complete transformation, and in the Polyclueta 

 are called genital tubes. 



As already explained, paired nephridia originally occur in all the 



Fig. 15S.— Transversa section tlirougli a trunk segment ol a oarniTorous Annelid, 

 diagrammatic. S, Setse ; ac, aoiculum (supporting seta) ; Im, longitudinal musculature ; vd, dorsal 

 vessel ; Ic, gill ; dc, dorsal cirrus ; dp, dorsal parapodium ; i>p, ventral parapodium ; vc, ventral 

 cirrus ; im, transverse muscles ; hm, ventral chord ; m, ventral vessel ; nil, circular musculature ; 

 ov, ovary ; np, nephridia ; tr, nephridial funnel ; iiid, mid-gut. In the body cavity are eggs. 



segments of the Annulate body, even in the cephalic or oral segment. 

 It may, however, happen that the nephridia in a smaller or greater 

 number of segments do not attain development. It is further very 

 generally found that some of the nephridia begin to form early in the 

 embryo or larva or young animal, and function as embryonic or larval 

 kidneys, but afterwards entirely disappear when the permanent 

 nephridia attain development. We shall call those which temporarily 

 appear in the ontogenetic development provisional or embpyonie 

 nephridia. We can again distinguish two sorts of such provisional 

 nephridia. (1) Those which appear in that region of the embryo or 

 larva which corresponds with the subsequent head segment, and lie at 

 the anterior end of the cell mass (mesoderm streaks), from which the 

 most important organs of the segmented mesoderm come ; these are 



