THE URO-GENITAL SYSTEM 367 



Taking now an animal of the type shown in Fig. 105, B, 

 with a continuous protocoele and with a metamerism marked 

 by several successive pairs of associated nephridia and gonads, 

 imagine the result of a gradual and equal expansion of the 

 latter until they attain the furthest possible limits (Fig. 105, 

 C; also Plate II). The protoccele becomes suppressed and in 

 its place exists a series of paired chambers, metacoeles, each 

 pair in contact with the previous and succeeding ones and en- 

 closing between them the alimentary canal. This latter part is 

 thus hung between dorsal and ventral partitions, the mesenter- 

 ies, each double and composed of the walls of the gonadic sacs ; 

 also each pair of cavities is separated from the next by similar 

 double partitions which form intersegmental diaphragms or 

 dissepiments. Each lateral metacoele opens to the exterior 

 by the opening which was once that of the gonadic duct. Thus 

 far no provision has been made for the nephridia, which, with 

 the suppression of the protocoele, find themselves deprived of 

 their original function. Their fate is, however, simple and 

 obvious, for they receive an investment of the gonadic wall, 

 and although lying between this and the outer body wall, 

 project into the metacoelic cavity, with which they communi- 

 cate through the ciliated nephrostome at their free end. But- 

 one further modification is necessary to adjust matters to 

 the new conditions, and that concerns the walls of the meta- 

 cceles. When in their original condition as the walls of 

 small sacs employed for the production of germ cells, every 

 portion of their surface is needed for the production of these 

 latter elements, but when expanded to their final dimensions, 

 they become mesenteries, dissepiments and the lining mem- 

 brane of body walls, and form a thin and firm membrane, the 

 peritoneum, while their original reproductive function is con- 

 fined to certain restricted areas, situated near the dorsal me- 

 senteries. These, by a slight evagination, produce rounded 

 elevations that project into the lumen of the cavity and form 

 specialized germ glands, the ovaries and the testes. Their prod- 

 ucts, when mature, separate from their place of origin and 

 wander freely about in the metaccelic cavity. From this they 



