2 SYSTEMATIC ARRANGEMENT 



B. Apoda. Foot none, or very rudimentary. 



III. Brachiopoda (Brachiopods or Lamp-shells). Mouth placed 

 at the base of two spirally-twisted ciliated arms between the 

 two leaves of the mantle, which are covered with two separate 

 shelly valves : they live attached to other marine bodies. 



IV. Pteropoda (Pteropods). Head prominent, with one or 

 rarely two pairs of fins on the side of the neck, by means of 

 which they swim on the ocean ; body often covered with a thin 

 conoidal shell. 



V. Cephalopoda (Cephalopods). Head distinct, large, with eight 



or ten or more arms, by which they walk head downwards. 



A. Pedifera, Gray. The adult animal furnished with a more or 

 less distinct foot placed under the body for crawling on the 

 ground or surface of the water. 



Class I. GASTEROPODA, Cuvier. 



Animal having a distinct head furnished with eyes and tentacles, 

 and usually protected by one or two unequal-sized valves, the largest 

 being conical, spiral, and enclosing the greater part of the body. 



Synopsis of Orders. 



Subclass I. Gills comb-like, formed of one or two series of lamellae 

 under the mantle on the back of the neck, or rarely round its 

 edges. Adult and larva shell-bearing ; larva furnished with 

 deciduous cephalic fins. Unisexual or hermaphrodite and self- 

 impregnating. Ctenobranchiata, Gray, 1840. Prosthobran- 

 chia (or Prosobranches), M.-Edwards, 1848. 



Order I. Pectinibranchiata. Gills comb-like, formed of one 

 (or rarely of two) longitudinal series of laminae on the left side 

 of the mantle over the back of the neck. Animal unisexual. 

 Shell spiral. 



Order II. Sctjtibranchiata. Gills consisting of two series of 

 lamellae, forming one or two series over the back of the neck or 

 under the edge of the mantle round the foot. Animal herma- 

 phrodite, self-impregnating. Shell conical, spiral or symmetrical. 

 Rectum often traversing the heart. 



Subclass II. Respiratory organs variously formed ; gills exposed or 

 only slightly covered by a fold of the mantle, or in the form of 

 a closed lung-like cavity. Hermaphrodite, with reciprocal im- 

 pregnation. Heterobranchiata, Gray, 1840. Opisthobranchia 

 (or Opistobranches), M.-Edw. 1848. 



Order III. Pleurobranchiata. Gills forming a tuft on the side 

 under a fold of the mantle. Animal hermaphrodite, with reci- 



