52 



MOLLUSCA. 



keel, with caudal gland and mucous aperture at end of body. A. cmpiricorum 

 Fe'r., Limax L. Kespiratory aperture behind the middle of the right edge of 



the mantle. Sexual opening far removed from respira- 

 ' tory aperture behind the right tentacle. Back keeled, 



without caudal gland and mucous orifice. L. agrestis 



L., L. cinereiis O. Fr. Mull. 

 Fam. Helicidae. Svccinea amphibia Drap., Pupa 



muscorum L., Clausilia Mdtns Drap.. Bulimus monta- 



nus Drap., Helix pomatia L. (Eoman snail), //. 



nemoralis L. 



Order 4. OPISTHOBRANCHIATA.* 



Hermaphrodite Gastropods, icith flat foot. 

 ^j Branchial veins open into the auricle behind 

 the ventricle. 



The great majority of this order are without 



FIG. 628. -Don. (Acan^o- * shelL The branchial Cavit 7 never contains 

 doris) piiosa (Bronn). Br, more than one gill. The gills are usually 

 giiis ; ^anus; F, tentacle. exposed (fig> ^) or absent. Sometimes there 



are dorsal processes, into which appendages of the alimentary canal 



enter (fig. 525). The nervous system contains cerebral, visceral and 



pedal ganglia (except in Tethys, which has 



a fused ganglionic mass and simple cesopha- 



geal commissure). The branchial veins, with 



a few exceptions (Gastropteron), enter the 



heart from behind. 



Sub-order 1. Tectibranchiata. Gill al- 

 most always on the right side, covered by 

 the mantle edge or placed in a dorsal 

 branchial cavity. Shell usually present 

 (fig. 524). 



Fam. Pleurobranchidae. With large gill on right 

 side, and usually internal shell. Pleurobranchcea 

 Mecliclii Cuv., Pleurolranchus aurantiacus Guv., 

 Umbrella meditcrranea Lam. 



Fam. Aplysiadae (Sea-hares). Shell covered by 

 two lobes of the foot. Aplysia depilans L., 

 Mediterranean. 



Fam. Bullidae. Bulla ampulla L., 

 aperta L., Accra bullata O. Fr. Mull. 



Br 



FIG. 524.- Pleurobranchus auran- 

 tiacus (regne animal). Br, 

 Philine gil i s . P> pen is ; F, tentacle ; 

 B, proboscis. 



Sub-order 2. Nudibranchiata. Marine Gastropods, without shell 



* Alder and Hancock, 1. c. H. Muller and C. Gegenbaur, " Ueber Phyllirhoe 

 bucephalum." Zeit.f. miss. Zool., Tom IV., 1854. 



