202 



IXVEIiTEBIiATE ANIMALS. 



Ktomacli, wliicli sometimes is prnvided with calcareous plates for 

 grinding down the f.iod. The intestine is long, and alwaj's ter- 

 minates in a distinet anal aiiertuie. J.listinet salivary glands are 

 usually present, and the liver is well developed. 



A distinct lieart (tig. lii!)) is ahiiost always present, and consists 

 of two chandier.s, an auricle and a ventricle. Resjiiration is very 

 \'ariou.'.!y effected — one great division being constructed to breathe 



Fix- 140.— A, Skc'tfh of a Wlu-lk (l;i„rl,r„M uihI,,!:,,,,) in iiiiitiiin: /Font; 7i Heail, 

 canyiii,^' tli,. frrh-is (/) willi tliu cy.-s (r) .'it tlieir bases ; /- rrol,,,sds : s Ros].iratory 

 sipliiiii or till"' l,y wliicli watrr is a.liiiiH,.',! to t.lie Kill.s; o Operculum. B Sliell of 

 tlie Wjii'lli ; „ Sjiiiv ; (; lin.ly-wlioil II Notcli in tlie front margin of the nioutli of 

 the sliell; ,„ onhT li]i .f the iiioutli of llie sliell. This llsure is half the natural 

 si/,^. (', A sieall eliisler of the eL;.L,'-eai.sules of the whelk, (li and C are afo-r 

 Wooilu'ar.l.) 



an- by nneiiis of water, whilst in another section the respiration is 

 aeiial. In the- former of tliesi — often spoken of as the "branchiate" 

 (Jasteropods— respiration may be carried on in three ways. Firstly, 

 there may be no special lircatliing-organ, the blood being simply 

 e.xpo.sed to tile action of the water, as it circulates through the thin 

 w.-ills of the mantle-e.avity. Secondly, the breatliing-organs may be 

 111 the form of outward jirocesses of the skin, exposed to view on 

 the back oi sides of the animal (tig. 144). Thirdly, tlie breathino-- 



