28o Introduction to Ajiimal Morphology. 



CHAPTER XXXVII. 



SUB-CLASS 3. — GASTEROPODA [Cuvter). 



Cephalophora, whose bilateral symmetry is often 

 hidden by the spiral coiling of the body. The undi- 

 vided mantle secretes a shell which consists of one, 

 two (rarely eight), pieces, antero-posteriorly, never 

 laterally, disposed. The head bears 1-3 pairs of ten- 

 tacles over the mouth. The foot is elongated, ventral, 

 muscular, often fitted for creeping (their commonest 

 mode of locomotion). This large class includes two 

 types of structure — ist, water-breathing forms with a 

 veliferous larv^a (Branchiata) ; and, 2nd, air-breathing 

 forms whose larva has no conspicuous velum (Pulmo- 

 nata). The former may have the gills behind 

 (Opisthobranchiata), or in front of the heart (Proso- 

 branchiata). 



The skin consists of a cuticle-like epithelium, 

 rarely ciliated entirely* (Phyllirhoe) in the adult, 

 always so in the larva,t and a connective basement 

 containing unstriped muscle ; sometimes with clear, 

 glossy, connective, or cartilage cells (Carinaria, Pte- 

 rotrachea), often calcareous^ concretions (Opistho- 

 branchiata§), unicellular pigment glands (Pulmonata), 

 and pouch-like mucous follicles, || as well as pigment 



* Cilia are widely distributed in Opisthobranchiata. The cilia are most 

 extensively spread where the gills are smallest. In others the cells are 

 cylindrical. 



t In Pontolimax a layer of cells underlies the cuticle like a deeper epi- 

 thelial stratum. 



X These often assume specifically characteristic shapes. 



§ In Doris these make a form of dermal skeleton. 



II In Elysia these are in pearly rows, and in oval groups in Gasteropteron. 



