196 IXVKltTKlUiATE ANIMALS. 



CHArTEi; XX. 



LIOLLI'SCA PROPER. 



TriE liigliei' J/olhixni m- Jfu/lnnm Pra/irr compri.se those members of 

 tlie suV)-kiiij;-dom ill which t/ir iirn-diix xi/xtem mit.iists nf t/o-ee prin- 

 rijiiil piilrx <if goiiiilio ; ami tliere ix ulirays a in'll-derehiped heart 

 tumxistuig >>f at Irast. tiro rhanihi'i'i^. 



In this ilivisioii :irc iiiclii(h'il tlie fullnwiiig classes: — 



1. Laiiiiilihntiii'lii'ita, witliiiut ii distiuct heail. 



i. Uaxt,-ri<l<n<hi, 1 



., „, , with a ilistiiict heail and a masticatory 



. ,, , , ■ I aiiparatus (ir oilniitoiihore. 



4, t '■^ili'itnpada^ ) ^ 



( 'l.,V.SW 1. Jj.MIELLIllHANCHIiVTA. 



Tliesc arc well kimwii as liivalve sliell-lish, .siicli as Mussels, 

 ( lysters, Scallnp.s, i*\:r., ,aiid they are all either marine or inhabitants 

 of fre.sli w.-itcr. They are distinguished from the other Molluscs by 

 having nil distinct head, and by lining the body protected by a 

 liiv.-dve shell ciinijiiisi'il nf two |iicces. They are called LamelUhraii- 

 i-liiatii friini the fact that the lug.'ins nf respiration are in the form of 

 leaf-like gills or branchia-', two of which (or sometimes one only) are 

 lilaced .it e.ich side of the body, constituting wh.at is known in the 

 (lystcr as thf "beard." The bmly of the LaindUhraadiiata is more 

 oi- less cniiiplctely ciicldscd in .-iii e.\|iaiision of the integument which 

 niiistitutes the "niaiitlc,'' and which is dixided into two halves or 

 "l.ibcs,'' whirh ai'e ]il;iccd nil the xiilrx iif the aiiiiii.al, and .secrete the 

 shell, 'bhe shell, therefnre, of the true Pivalves is comjiosed of two 

 v.alve-f, v.]i\i-\\ are "right" and "left," and nnt "dorsal" and "ven- 

 tral " as ill the Jli-a,-l,i'ni„„hi. Mm-eover, the v.-dves of the .shell are 

 usually nf the s.-iiiic size, so 1li;it the shell is " eijiiivah-e " ; and, 

 lastly, the shell is iiinrc de\-elii|icd on nne siile than the other, so as 

 ti. become "iiie(|iiil;i1rral" dig. l:',.s). The lobes of the mantle are 

 sonietiines (|iiilc five .-ilong their \-ciilr,-il ni:n-giii ; but at other 

 limes (liry aiv iiioic or ll■s^ united to each other, :iiid leave oiilv two 



