V. CEPHALOPODA: SUMMARY OF IMPORTANT FACTS. 397 



23. The SCAPHOPODA are primitive forms with tubular shells. 



24. 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. 



25. The mantle cavity contains one or less frequently two 

 ctenidia, or these may be degenerate and a lung may occur. 



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

 the gonads, always unpaired, are hermaphroditic or dioecious. 



27. The shell is always unpaired; it is usually coiled in a (right- 

 hand) spiral, and is frequently closed with an operculum. 



28. According to characters derived from nervous system, 

 sexual organs, heart, and respiratory organs the Gasteropods are 

 divided into (1) Prosobranchia; (2) Opisthobranchia: and (3) 

 Pulmonata. 



29. The Opisthobranchia are hermaphroditic; orthoneurous; 

 have gills of various kinds (or none), and have the auricle always 

 behind the ventricle; shell and mantle reduced or absent. 



30. The Pteropoda are pelagic Opisthobranchs with wing- 

 like processes of the foot and frequently reduced shell or none. 



31. 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. 



32. The Heteropoda are pelagic Prosobranchia with foot 

 divided into fin and tail, shell rudimentary, or naked. 



33. The Pulmonata are in some respects (orthoneurous and 

 hermaphroditic) Opisthobranch-like; in other respects as in posi- 

 tion of heart, development of shell and mantle like the Proso- 

 T^ranchs; the mantle cavity functions as a lung. 



34. The CEPHALOPODA have no true foot; but its homologues 

 are to found in the siphon and in the tentacles, usually provided 

 with suckers, on the head; they have an unpaired mantle and a 

 single shell or none. 



35. The unpaired mantle cavity contains one or two pairs of 

 'ctenidia. The water is forced from the mantle cavity through 

 the siphon. 



36. The number of auricles corresponds with the number of 

 ctenidia; besides the systemic heart there are one or two pairs of 

 branchial hearts, elsewhere unknown in molluscs. 



37. The sexes are separate. 



38. The ink sac is peculiar to Cephalopoda. 



39. The eye is (usually) highly developed (with retina, choroid, 



