GASTEROPODA. 9 



a kind of lid or operculum to the aperture of the 

 other valve. 



Sub-class I. Gill comblike, formed of one or two 

 series of lamella, under the mantle on the back 

 of the neck, or rarely round its edge, in an 

 open gill cavity. Adult and larva shell- 

 bearing; larva furnished with deciduous cepha- 

 lic Jins. Unisexual or hermaphrodite, and self- 

 impregnating : Ctenobranchiata. 



Order I. PECTINIBRANCHIATA. Gills comblike, 

 formed of one (or rarely two) longitudinal 

 series of lamina?, or rarely of branched ves- 

 sels, on the left side of the back of the mantle 

 over the back of the neck. Animal unisexual; 

 shell spiral. 



Order II. SCUTIBRANCHIATA. Gills consisting 

 of two series of lamella?, forming one or two 

 series over the back of the neck, or under 

 the edge of the mantle round the foot. Ani- 

 mal hermaphrodite, self-impregnating; shell 

 conical, spiral, or symmetrical ; rectum often 

 traversing the heart. 



Sub-class II. Respiratory organs variously formed, 

 gills exposed, or only slightly covered by a 

 fold of the mantle, or in the form of a closed 

 lunglike cavity. Hermaphrodite, with reciprocal 

 impregnation : Heterobranchiata. 



Order III. PLEUROBRANCHIATA. Gills forming 

 a tuft on the sides under a fold of the mantle. 



