ON THE PAIRED NEPHRIDIA OF PROSOBRANCHS. 615 
of either ctenidium at the end of a papilla (P.). The rectum 
(R.) passed through the pericardium (Pc.), but ventrally from 
the heart, without perforating the ventricle (V), opening by the 
Diagram of the reno-pericardial system of the ancestral form of molluscs. 
ao. Aorta. az. Auricles. Ma. Mantle. The other letters are ex- 
plained in the text. 
anus (A.) in the middle line, at the bottom of the mantle 
cavity (M). Most probably the entire anterior half of the epi- 
thelial lining of the pericardium, that is to say, the portion 
anterior to the region occupied by the heart, represented the 
gonad (G.). The sexual products dropped into the cavity of the 
pericardium passed through the funnel into the nephridium, 
and passed out of the nephridial duct into the mantle cavity. 
This condition would be nearly realised by the actual Neo- 
meniz. 
Very soon the portion of the pericardium, the lining of 
which represented the epithelium of the gonad, became con- 
stricted off the pericardium (compare the development of 
Paludina). This is the case in all other groups of Mollusca 
except Neomeniz. 
According to Pruvot, the gonad in Aplacophora is 
formed by two symmetrical portions only separated by a ridge. 
The gonad is unpaired in Placophora, Cephalopods, and 
Gasteropods. These certainly being less modified forms 
than Lamellibranchiata and Scaphopoda, where the 
gonad is paired, it is reasonable to infer that the original 
