34S MOLLUSCA 



13. The ArrruAi^v lack head ami cephalic appendages. 



14. The\- are l>ilaierall\- svinmelrical and ha\e paired organs: mantle 

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



i^. Iir man\- .\cephahi [Asiphoiiid) Uie mantle folds are completely 

 separated wntr.iUy. 



10. In [\\cSify!u'iijhi the lower edges of the mantle are uriiled, Iea\-ing 

 three openings: vTi in front for the loin : ij) hehindand l)elow,the hranchial 

 sijihon for the ingress of water and nourishment: (0 behind and ahoxe, 

 the ai\al or excurreirt siphon for the water used hy the gills aiul the 

 f.eees. 



17. There are two jxiirs of gills (ctenidia), wliich maybe combdike, 

 filiform, or most conrmonly lamellar (lamellibranchs). 



iS. Correspondingly the heart has two auricles; the unpa.ired ventricle 

 is usually tra\""rsed by the reclunr. 



to. The foot is a compiressed muscular mass frequently pro\ided 

 with a livssus 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 {rrotocoiiclia) have primitive gills and hinge; 

 others {Hclcroi'oih'lia) are more highh" developed. 



22. The Sc.\PiiCPOD.\ are primitive forms with tubular shells. 



23. The G.\STEKOPOD.\ (Cephalophora, or snails) ha\e a distinct 

 head bearing eves and tentacles: a creeping foot, an uirpaireil mantle 

 (occasionall}- absent"), and a univalve shell or none. 



24. The mantle cavitv contains one or less frequentlv two ctenidia, 

 or these mav be degenerate anil a lung may occur. 



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

 the gonads, always unpaired, are hermaphroditic or diivcious. 



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

 spiral, and is frequently closed by an operculum. 



27. According to character of ner\ous system, sexual organs, heart, 

 and respiratory organs the Gasteropods are dixided into (1) rrosobranchia; 

 (2) Opisthobranchia; and (3) Pulmonata. 



28. The 0[>islliabranihid are hermaithroditic; orthonenrous; haxe 

 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 fre(|uentl}- reduced shell or none. 



30. The Prosohranchia ha\e the gills (ctenidia — occasionally paired) 

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

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



