170 THE INFLUENCE 01' INANIMATE SUEROUNDINGS. 



some species of Lucina (fig. 48, a), situated on the ventral 

 margin of the mantle. In the Annelida the gills are usually 

 an appendage of the legs, and sometimes are placed directly on 

 the body or at the fore end, as in Sahella, Strpula, Terehdla, 

 &c. Finally, the number of Invertebrata is by no means small 

 ■which dispense entirely with such distinct, conspicuous organs 



:==r^ 



Pig. 43. -Gills of Mollusca. a, Lvcim pldlippensis, with four nmntle gills belnnd the 

 muscle m ; i, Jlytilus, with k the gills, and I the laminated lip ; c, Elysia grandli 

 (Bergh.), destitute of gills ; d, Doris sp., with a taU tuft of dorsal branchiffi. 



of respiration, and consequently breathe only through the skin ; 

 among the Mollusca there are the Elysiadce (fig. 48, c) and their 

 allies; among the Annelida the common leech and the Oligo- 

 ch£Eta3 (the earth-worm, &c.); many of the lower Crustaceans — 

 parasitical as well as independent — all Infusoria, the Coelente- 

 rata, and even many Echinodermata, &o. 



