328 MOLLUSCS. 



much reduced, the odontophore is feebly developed. Chietodervia is 

 more elongated and cylindrical ; its shell is represented by minute 

 spines, its mantle is reduced, the foot is abortive, there are onlytwo 

 gills, the odontophore is very slightly developed, and has a radula vi^ith a 

 single tooth. The last three forms have been dredged from considerable 

 depths. They suggest in some ways how Gasteropods may be linked to 

 "worm " types. 



Sub-Class II. Gasteropoda Anisopleura. 



Gasteropods which are more or less asymmetrical. The head and 

 foot remain bilateral, but the visceral hump with its mantle fold is 

 twisted round to the right and forwards. Thus the anus, the gills, the 

 nephridia, and the genital aperture lie on the right side, and more or 

 less anteriorly. The originally right gill comes to lie to the left, and 

 the originally left gill usually atrophies. A similar fate seems to befall 

 the originally right nephridium. The main torsion must be distinguished 

 from the spiral twisting which the visceral hump often exhibits, and 

 from the associated spiral coiling of the shell. Moreover, a super- 

 ficial bilateral symmetry is sometimes acquired by free-swimming forms, 

 e.g. , Heteropods. The reproductive organ and genital duct are single. 

 There is usually a pedal gland in the foot. 



I. Streptoneura. Visceral nerve loop twisted ; sexes separate. 



(1) Zygobranchs. Both gills present or absent. A rudimentary 



nephridium sometimes persists on the left side. Small 

 visceral dome. 

 Examples. — Patella (limpet), Haliotis (ear-shell). 



(2) Azygobranchs. Originally right gill and left nephridium 



persist. Large visceral dome and shell. 

 Examples — 



Pond-snail (Paludina) ; whelk (Littorina) ; buckie 

 {Buccinum); dog- whelk (/'a^jSara); cone-shells (Co««/); 

 Murex; lantkina. 



Also the pelagic Heteropods, with foot adapted for 

 swimming, e.g., Atlanta (large shell), Carinaria 

 (small shell), Pterotrachea (no shell). 



II. Euthyneura. Visceral nerve loop not twisted, often short ; her- 



maphrodite ; shell often light and lost in adult, 

 (i) Opisthobranchs. Small visceral hump ; rarely any shell in 

 the adult ; anus behind and heart in front of the gill. 

 Examples — 



(a) With developed mantle fold, usually with delicate 

 shell (Tectibranchia), e.^., Aplysia, Bulla, Phyl- 

 lidia. 

 {b) With atrophied mantle and no shell on adult (Der- 

 matobranchia or Nudibranchs), e.g. , Doris, Eolis 

 Phyllirhoe. ' 



(2) Pulmonata. Air-breathing, without gill, with respiratory 

 mantle-cavity, e.g., Helix (snail), Limax (grey slue) 

 Anon, black slug ; Linma:ns, Planorbis, and Ancylm 

 (common fresh-water snails). 



