no 



THE GASTROPODA 



communication. Elysia is exceptional in that the kidney is placed 

 below and partly surrounds the pericardium, and the reno-pericardial 

 orifices are multiple, some ten being present (Fig. 92). As a rule 

 the external opening of the kidney is situated near the anus (Figs. 

 81 and 88), and sometimes the two open together into a sort of 

 common cloaca, as may be seen in the Gymnosomata (Fig. 84, IX) 

 and in certain Pulmonates, such as Lirnax (Fig. 86, I), the Onci- 

 diidae (Fig. 59), and Vaginula (Fig. 87), but not in V. willeyi. In 

 rare cases, however, such as the Nudibranch. Janus, the excretory 

 aperture is distant from the anus. The external renal orifice is 



borne on a papilla in various 

 Aspidobranchs with two kidneys 

 (Fig. 88, /), but is a simple slit, 

 shaped like a button -hole, in 

 the majority of Pectinibranchia 

 (Fig. 99, IV) and Tectibranchia 

 (Fig. 154, o). Among the Pec- 

 tinibranchs, however, Paludina 



Fio. 91. 



Diagram of the two renal organs of 

 Patella, to show their relations to the 

 rectum and to the pericardium. /, papilla 

 of the larger kidney ; <j, anal papilla, 

 with rectum leading from it; h, papilla 

 of the smaller kidney, which is only 

 represented by dotted outliiu-s ; I, peri- 

 cardium, indicated by a dotted outline 

 (at its right side are seen the two reno- 

 pericardial pores) ; /, the sub-anal tract 

 of the large kidney given off near its 

 papilla and seen through the unshaded 

 smaller kidney ; ks.a, anterior superior 

 lobe of the large kidney ; ks.l, left lobe 

 of same ; ks.i, inferior sub-visceral lobe 

 of same; ks.p, posterior lobe of the 

 right kidney. (After Lankester.) 



Fio. 92. 



Elysia viridis, heart and kidney, 

 dorsal aspect (somewhat schematic). 

 I, ventricle of heart ; II, external renal 

 pore ; III, auricle ; IV, kidney ; V, the 

 various reno-pericardial pores on the 

 left side (there are tive such pores on 

 the right-hand side); VI, the ventral 

 reuo- pericardia! pore ; VII, pericardium. 



and Valvata are exceptional in possessing an ureter which opens at 

 the edge of the mantle. The same arrangement is found in many 

 Pulmonata, especially in the Stylommatophora, in which an elongated 

 ureter opens alongside of the anus at the margin of the pneumo- 

 stome (Fig. 86, V). 



As regards its structure, the kidney in its simplest form is a 

 sac lined by a secretory epithelium. By the infolding of its walls, 

 the cavity of the sac is subdivided and the organ acquires an 

 alveolar structure of spongy appearance, but in various pelagic 

 forms it again becomes more or less tubular and transparent, e.g. in 

 the Heteropoda (Fig. 141, q\ in certain "Pteropoda" (Fig. 60, fc), 



