244 Introductio7i to Animal Morphology. 



laterally symmetrical, but from unilateral growth 

 often assumes a spiral form. Shells increase in ex- 

 tent by additions secreted by the thick and glan- 

 dular margin of the mantle, in the form of long 

 prismatic cells, or minute, conical shell-columns. 

 The surface of the mantle secretes the nacreous or 

 mother of pearl layer, 

 which increases the 

 thickness of the shell, 

 and consists of thin 

 overlying plates, whose 

 wavy, out - cropping 

 edges, viewed by re- 



flc^nf^rl lio-lTf ?\r\r\(^7\r iri A, transverse section of Bivalve Mollusc ; 



neCtea llgnt, appear iri- ^^ jji^^g . e, foot ; B, double gill lamella : 



J , T'ViQ /-Mifoirl/^ /-w-T the shaded outer layer is the shell, the clear 



UeSCent. ine OUtSiae OI spaceintemaltoitisthcmantle. B, section 



,, ,,,. ^, ., of Gasteropod ; B, gills ; E, foot. 



the shell is coated with 



a fine periostraaim, a horny cuticle often covered with 

 hair-like or laminated processes. The shell may de- 

 velop internal processes, as in Brachiopoda, but it is 

 always an epidermal structure. An internal skeleton 

 sometimes exists as a series of symmetrical, simple,, 

 cellular cartilages around the pharynx or oesophagus, 

 but not homologous with any specific part of the ver- 

 tebrate endo-skeleton. The muscular system is, as a 

 rule, feebly developed, though special parts of it, 

 modified by the forms of the shell and foot, may be 

 very strongly marked. The nervous system consists 

 of a pharyngeal ring, having a pair of epi- and a pair 

 of hypo-pharyngeal ganglia united by commissures ; 

 of these, one or other may preponderate, or they may 

 become fused or sub-divided, or their commissures 

 may be short or long. The hypo-pharyngeals are 

 often displaced downwards into the foot, and hence 



