552 H. p. KJERSCHOW AGERSBORG 



becomes constricted off from the main division of the coelom 

 (the pericardium), forming a small genital sac. From the wall 

 of this sac the genital duct grows out, and joins an epidermal 

 invagmation like the peritoneal funnel of the right side.' In 

 Melibe the gonads are situated ventrally with openings on 

 the right side ; the single kidney (nephrocoel) is situated 

 dorsally and communicates internally with the pericardium 

 through the renal syrinx and externally through the ureter 

 on the left side of the anal pore. The perigonadial coelom 

 (perigonadium) and pericardium are completely separated by 

 the visceral cavity or vascular (haemocoels) cavities. These 

 cavities are blood-spaces into -which the blood percolates from 

 the atrial A'essels bathing the visceral organs. According to 

 Goodrich (1895), in Chitons, a separation has taken place 

 in the genital region of the coelom from the renal ; the gonads 

 then require special ducts which may not be homologous with 

 the peritoneal funnels. InPeripatus, soon after the meta 

 meric somites have been hollowed out from the coelomic 

 follicles, the upper half of each coelomic cavity ])ecomes nipped 

 off from the lower half. From the wall of each of these lower 

 coelomic sacs a peritoneal funnel is formed as an outgrowth 

 which fuses with the epidermis. While these organs have 

 developed in this way, the dorsal or genital halves of the somites 

 in the posterior segments have become fused, forming two 

 genital tubes communicating posteriorly with the undivided 

 coelomic follicles of the last segment. The peritoneal funnels 

 of this segment retain their primitive function and develop 

 into the genital duct ; Sedgwick (1898 : 375) finds that the 

 coelom of the G a s t e r o p o d a is in three sections. (1) The peri- 

 cardium ; (2) the nephridia ; (3) the gonads. The pericardium 

 is in relation with the heart ; it normally communicates with 

 the nephridial system, and part of its lining is generally 

 glandular and forms the pericardial gland. It has no con- 

 nexion with the blood system. Finally, Lankester (1893 : 

 428) points out the following : the perigonadic spaces and the 

 pericardial space are, then, the coelom of the Mollusc a. 

 It is quite distinct from the haemocoel. In cephalopods, and 



