306 Introduction to Animal Morphology. 



21. Aplustridae shell perfect; lateral processes of head lobes 

 ear-like ; teeth 13. o 13. 22. Actaeonidae shell perfect, 

 operculigerous, involute: teeth oo . o. oo or 12. o. 12. The 

 shell-less Dipleurobranchs include (22) the toothless Phylli- 

 diidae, and (23) the jaw- and tongue-bearing Pleurophyllididae, 

 with lateral anus. 



Sub-order 3. Heteropoda(CWVr) dioecious, marine, trans- 

 lucent ; foot compressed, fin-like ; shell thin or none ; viscera in 

 a nucleus ; gills pectinate or filamentary ; propodium vertical, 

 fin-like ; mesopodium suctorial in the males of some, the 

 sucker having special circular and radiating fibres.* Family i. 

 Pterotracheidae body long; nucleus small, often stalked, 

 naked (Pterotrachea, Firoloidea),f or with a shell (Carinaria 

 and Cardiapoda) ; gills tufted, pre-nuclear ; tentacles none 

 (Pterobranchea), or present. 2. Atlantidae, shell discoidal, 

 spiral, operculigerous, into which the animal is retractile.^ 



Sub-order 4. Prosobranchiata (Milne Edwards),^ includ- 

 ing shelled Gasteropoda, which have the gills in front of the 

 heart, and a veliiVrous larva. The visceral cavity is large, 

 and the body nearly symmetrical. The following sections 

 are included : 



I. Polyplacophora (Blainville) chitons depressed, ovate, 

 with a broad foot and a shell of eight valves. The mantle border 

 is smooth or with excrescences ; monoecious (Middtndorf) ; 

 cyclobranchiate ; teeth 4-6. 2. 1.2,4-6 on a long radula ; 

 intestine straight or coiled ; anus posterior, median ; tentacles 

 brain and eyes none ; heart medio-posterior, elongated ; sex 

 organs paired, with two openings. The shell in Cryptoplax, 

 &c., is imbedded in the mantle. The anterior and posterior 

 valves are semicircular ; the six intermediate have each three 



* And between these are long radial glands. 



t The larva of this genus has two gizzard-plates in the stomach. The 

 peculiarities of Heteropods come out late in Embryogenesis. 



\ Bellerophon, Eccyliomphalus, and Cyrtolithes are fossils allied 

 thereto. 



The number of species in each group of Gasteropoda is as fol- 

 lows : Opisthobranchia, 825 living, 335 fossil. Heteropoda, 54 living, 

 191 fossil. Abranchiata, 25 living, none known fossil. Prosobranchiata, 

 8600 living, 5670 fossil. Pulmonata, 5800 living, 530 fossil in all, 21,980. 



