ON THE STRUOTtJRG OP .ENIGMA ^-NIGMATICA. 277 



where it forms a large lobe above and to the right of the 

 intestinal loop. From this lobe two main branches pass 

 backward. The one passes to the dorsal side of the ctecum 

 of the crystalline style, and accompanies the caecum for the 

 whole of its course in tlie right mantle lobe, and forming a 

 large mass above its extremity, as shown in fig. 13. The 

 other branch, into which the gonaduct opens, maintains a 

 more dorsal position, and, passing above the adductor muscle, 

 forms a considerable follicular mass to the right of and above 

 the posterior part of the intestine. 



The gonaducts, as has been described above, open by 

 distinct ciliated funnels into the renal sacs close to the reno- 

 pericardial apertures of the sides of the body to which they 

 belong. A gonaducal funnel of a male is shown in fig. 15. 

 There is no difference in the histological characters of the 

 funnel in the two sexes. The gonaducts, that is to say those 

 sections of the ovarian or testicular tubules that are lined by 

 a low cubical instead of a germinal epithelium, are extremely 

 short. The histology of the ovaries and testes does not call 

 for special description, but in one of my series of sections the 

 ovai-y was penetrated throughout by a number of green 

 filaments, whose nature I could not satisfactorily determine. 

 They have the appearance of filamentous algce, and may 

 possibly be symbiotic or parasitic within the mollusc. But 

 they are not of constant occurrence, for I could find no trace 

 of them in two other females examined. 



The Nervous System. — As may be seen in fig. 3, the 

 nervous system is of the usual lamellibranchiate type, and 

 the modifications it has undergone are attributable only to 

 the torsion which has affected these in common with all the 

 other organs of the anterior moiety of the body. Thus the 

 right cerebro-pleural ganglion lies above and somewhat 

 behind the left. The cerebro-pedal connectives are relatively 

 short, and the right pedal lies above the left pedal ganglion. 

 The right visceral connective passes above, and the left 

 visceral connective below, the byssus muscle and the right 

 visceral ganglion is somewhat above and in advance of the 



