MOLLUSCA. 121 



Class II. Gastropoda. 



The foot usually forms a simple sole-like ventral expansion. 



A. GASTROPODA ISOPLEUBA. Forms exhibiting bilateral sym- 

 metry in the external form and in the arrangement of the auricles, 

 ctenidia (gills), nephridia, genital ducts and nerve-cords. Anus posterior. 

 Ganglia ill-developed. 



Order 1. Solenogastres (Amphineura). Marine worm-like Molluscs 

 with foot absent or rudimentary, and the body covered by calcareous 

 spicules. Proneomenia. Neomenia. Chaetoderma. 



Order 2. Polyplacophora. Eight calcareous plates along the back. 

 Chiton squamosus. 



B. GASTROPODA ANISOPLEURA. Head and foot bilaterally sym- 

 metrical, but the visceral hump and mantle have undergone twisting 

 so that the anus, nephridial opening, and genital opening are shifted 

 more or less to the front. Generally a univalve shell, and a single 

 auricle, ctenidium, nephridium, and genital duct. 



Order 1. Streptoneura (Prosobranchia). Visceral nerve-loop twisted ; 

 gills in front of the heart ; sexes separate. Patella, limpet (both 

 ctenidia aborted, secondary gills), Fissurella and Haliptis with both 

 ctenidia and two auricles ; all three with paired nephridia. Trochus 

 (rhipidoglossate). Janthina ptenoglossate). Murex, Buccinum, whelk, 

 and Purpura lapillus (rhachiglossate). Conus (toxoglossate). The fol- 

 lowing are taenioglossate : Cyclostoma elegans (lung-breathing), Palu- 

 dina vivipara, river-snail, Valvata (^ ) all fresh water : Cypraea, 

 cowry, Strombus ; Entoconcha mirabilis, parasitic in Synapta ; Am- 

 pullaria, with gill- and lung-chamber. 



These forms are closely allied to the pelagic taenioglossate Heteropoda 

 (Natantia) with fin-like foot : Bellerophon, fossil ; Carinaria, Atlanta. 



Order 2. Euthyneura. Visceral nerve-loop not twisted. Hermaph- 

 rodite. 



(a) Opisthobranchia, genital openings on the side of the body ; 

 the branchial veins open behind the ventricle into the auricle ; larva 

 with operculate shell, and velum. Shell usually absent in the adult : 

 Aplysia, Pleurobranchus, Bulla (tectibranchs) : Phyllirhoe, Tethys, 

 Doris, .ffiolis, Rhodope (nudibranchs). Elysia (saccoglossate). 



(b) P u 1 m o n a t a, hermaphrodites with a lung-chamber : Pul. basom- 

 matophora, eyes at base of two non-invaginable tentacles: Lymnaeus 

 stagnalis, pond-snail, L. tmncatulus, Planorbis corneus, trumpet- 

 snail. Pul. stylommatophora, eyes at tips of two invaginable tentacles : 

 Arion ater, black slug ; Limax agrestis, garden slug, L. flavus, cellar 

 slug; Helix aspersa, garden snail, H. pomatia, Koman snail, H. 

 nemoralis, hedge snail. 



