30 



piratory organs selected to characterize the subdivision of the class into seven 

 Orders, as follows : — the first group, Pect'mibrancJdata, have a pair of pecti- 

 nated gills of the shape of a comb ; the second, Pulmobranchiata, which 

 includes the terrestrial and lacustral species perform the functions of respira- 

 tion by a pulmoniferous net-bag ; the tliird, Pletwobranchiata, are distin- 

 guished by the giUs being situated on the right side only ; the fourth, Cervico- 

 hrancMata, have their gills situated in a special cavity in the neck ; in the 

 fifth, Ct/dobranehiata, the giUs are placed in a circle round the edge of the 

 body ; in the sixth they are cirrous or hair-Kke ; and in the Nucleobranchiata, 

 which are the most aberrant form of the class and afford a link to that 

 of the Pteropods, the braucliise, together with the visceral mass, are con- 

 centrated within a nucleus protruding from the ventral part of the body 

 which is ahnost wholly gelatinous. Numerous modifications of the breathing 

 organs of minor importance have been selected for the distinction of orders ; 

 those which have the gills ranged around the lower part of the body, 

 between the disc and the mantle, have been associated under the term 

 Inferobratichiata, those in which the gills and entire body are contained 

 in a strictly tubular shell, TubidibrancJnata, and those in wliich they are 

 protected by a shield-like shell under that of Sctitibranchiata ; aU these are, 

 however, comprehended in the seven Orders here adopted. 



Order 1. PECTINIBRANCHIATA. 



Branchiae; pectinated, contained within a cavity in the upper j)art 

 of the neck, into which the water is sometimes conveyed by a 

 siphon or siphonal appendage. 



Having already spoken of the nature and habits of the pectinibranchial 

 Gastropods in treating of the class, I have only to remark under tliis head 

 that the branchise or gills of the present order, wliich includes a very 

 numerous series, are arranged for the most part in parallel laminse, like the 

 teeth of a comb, and are contained within a cavity in the upper part of the 

 neck. Some of the pectinibranchial tribe are carnivorous, and some are 

 herbivorous ; and the former are distinguished fi'om the latter by a siphon 

 or siphonal appendage passing out at the basal channel of the shell, for the 

 purpose of convejing the water to the branchial cavity. 



Carnivora. Herbivora. 



CoNvoLUTA Alata Turbinacea Neritacea 



COLUMELLATA CaNALIFERA PlICACEA PeRISTOMATJ 



PURPURIFERA PARASITICA IaNTHINEA MeLANIANA 



