4OO 



MOLLUSCA. 



gills. The segmentation is holoblastic, and a gastrula is formed by 

 invagination. 



FIG. 200. Anatomy of Chiton. 



A, ventral surface (after Cuvier). B, dorsal view of alimentary 

 canal (after Lankester). C, genital and excretorj^ organs from 

 dorsal surface (after Lang and Haller, diagrammatic). ;., 

 mouth ; a., anus ; dr., numerous simple gills ;f., foot ; b., buccal 

 mass ; /. , liver ; i., intestine ; ao., aorta ; v., ventricle of heart ; 

 r.a. and /.., right and left auricles ; ov., ovary ; od., ovidu> t ; 

 od'., opening of oviduct ; n., part of nephridium, represented in 

 black throughout; no., external opening of nephridium; /. , 

 outline of pericardium. 



m 



Sub-class II. GASTEROPODA ANISOPLEURA, e.g. 

 Snail, Whelk, Limpet. 



In these more or less asymmetrical Gasteropods, the head 

 region, which is well developed, remains symmetrical, and so 

 does the foot, which is typically a fiat creeping organ. But 

 the visceral mass or hump, with its mantle fold, is more or less 

 twisted forwards and to the right. Thus the pallial, anal, 

 nephridial, and genital apertures usiially lie on the right side, 

 more or less anteriorly. A further asymmetry is shown by 

 the twisting of the morphologically right gill to the left side, 

 while the original left gill is usually lost. Similarly, one of 

 the nephridia, probably that which is morphologically the left, 

 tends to disappear, and in most cases only one persists 

 topographically on the left side. The main torsion must be 

 distinguished from the spiral twisting which the visceral 

 hump often exhibits, and from the frequently associated spiral 

 coiling of the univalve shell. Moreover, a superficial secondary 

 bilateral symmetry tends to be acquired by free-swimming forms, 



