34S MOLLUSCA 



13. The ACEPHALA lack head and cephalic appendages. 



14. They are bilaterally symmetrical and have paired organs: mantle 

 folds, bivalve shell, gills, nephridia, and gonads. 



15. In many Acephala (Asiphoma) the mantle folds are completely 

 separated ventrally. 



16. In theSiplionata the lower edges of the mantle are united, leaving 

 three openings: (i) in front for the foot; (2) behind and below, the branchial 

 siphon for the ingress of water and nourishment; (3) behind and above, 

 the anal or excurrent siphon for the water used by the gills and the 

 fa?ces. 



17. There are two pairs of gills (ctenidia), which maybe comb-like, 

 filiform, or most commonly lamellar (lamellibranchs). 



1 8. Correspondingly the heart has two auricles; the unpaired ventricle 

 is usually traversed by the rectum. 



19. The foot is a compressed muscular mass frequently provided 

 with a byssus gland. 



20. The shell consists of cuticular, prismatic, and nacreous layers. It 

 is closed by one or two adductors and opened by an elastic ligament. 



21. Some Acephals (Protoconc/ia) have primitive gills and hinge; 

 others (Heteroconcha) are more highly developed. 



22. The SCAPHOPODA are primitive forms with tubular shells. 



23. The GASTEROPODA (Cephalophora, or snails) have a distinct 

 head bearing eyes and tentacles; a creeping foot, an unpaired mantle 

 (occasionally absent), and a univalve shell or none. 



24. The mantle cavity contains one or less frequently two ctenidia, 

 or these may be degenerate and a lung may occur. 



25. Nephridia and auricles are rarely paired (with paired gills); 

 the gonads, always unpaired, are hermaphroditic or dioecious. 



26. The shell is always unpaired; it is usually coiled in a (right-hand) 

 spiral, and is frequently closed by an operculum. 



27. According to character of nervous system, sexual organs, heart, 

 and respiratory organs the Gasteropods are divided in to (i)Prosobranchia; 

 (2) Opisthobranchia; and (3) Pulmonata. 



28. The Opisthobranchia are hermaphroditic; orthoneurous; have 

 secondary gills (or none), and have the auricle always behind the ventricle; 

 shell and mantle reduced or absent. 



29. The Pteropoda are pelagic Opisthobranchs with wing-like pro- 

 cesses of the foot and frequently reduced shell or none. 



30. The Prosobranchia have the gills (ctenidia occasionally paired) 

 far in front, and in consequence the auricle in front of the ventricle ; they 

 are streptoneurous and dioecious; the mantle and shell well developed. 



