36 INTRODUCTION TO CLASSIFICATION. 



which is formed as an excretion from the surface of the lobes of 

 the mantle, and is composed of layers of animal matter hardened 

 by deposit of carbonate of lime, which may or may not take a 

 definite form, and so give rise to " prismatic " and " nacreous " 

 substance. As the lobes are right and left, so the valves of the 

 shell are right and left, and differ altogether from the valves of 

 the shell of the Brachiopoda. which are anterior and posterior. 

 The valves of the shell ca.n be brought together bv adductor 



O O v 



muscles. Of these one (Fig. 13, IT) always exists, posteriorly, 

 on the neural side of the intestine. A second (Fig. 13, J) is 

 commonly found anteriorly to the mouth, on the haemal side of 

 the intestine. 



XII. THE BRANCHIOGASTEROPODA. 



The hiatus between the present class and that just defined is 

 considerable, though not quite so well marked as that between 

 the Ascidians and the Lamdlibrancliiata. This group, which 

 contains the whelks, periwinkles, sea-slugs, or the Heteropoda, 

 Peetinibranchiata, Cycldbranchiata, Tectibranchiata, Inferobran 

 chiata, and Nudibranchiata, of Ctivier, consists of animals which, 

 like the Lamellibranchs, possess (in their young state, at any 

 rate) a mantle ; a foot, which is the chief organ of locomotion ; 

 and three principal pairs of ganglia cerebral, pedal, and 

 parieto-splanchnic. When they are provided with a heart, 

 which is usually the case, it is divided into auricular and ven- 

 tricular chambers ; but the mantle, instead of being divided into 

 two lateral lobes, is continuous round the body, and when it 

 secretes a shell from its surface, that shell is either in a single 

 piece, or the pieces are repeated from before backwards, and 

 not on each side of the median line. The shell of a Branchio- 

 gasteropod may, therefore, be univalve, composed of a single 

 conical piece, straight or coiled; or it may be multivalve 

 formed of a number of segments succeeding one another antero- 

 posteriorly ; but it is never bivalve. 



Sometimes a shelly, horny, or fibrous secretion takes place 

 from the foot, giving rise to a structure resembling the byssus 

 of some Lamellibranchs ; it is the so-called " operculum," which 



