110 THE GASTROPODA 
communication. lysiw 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). Asa 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 Limaz (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, 7), but is a simple slit, 
shaped like a button-hole, in 
the majority of Pectinibranchia 
(Fig. 99, IV) and Tectibranchia 
(Fig. 154, 0). Among the Pee- 
tinibranchs, however, Paludina 
hest 
Fic. 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; g, anal papilla, 
with rectum leading from it; h, papilla 
of the smaller kidney, which is only 
represented by dotted outlines ; l, peri- 
cardium, indicated by a dotted outline 
(at its right side are seen the two reno- 
Fic. 92. 
pericardial pores) ; ff, 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./, left lobe 
of same; ks.i, inferior sub-visceral lobe 
of same; ks.p, posterior lobe of the 
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 five such pores on 
the right-hand side); VI, the ventral 
right kidney. (After Lankester.) reno-pericardial 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, ), 
