CLASSIFICATION OF MOLLUSCS 473 



rounded bv a diffuse digestive gland. There is a radula in the mouth. 

 The heart is median and posterior, and consists of a ventricle and 

 two to eight auricles. There are two symmetrical nephridia opening 

 posteriorly, and consisting of much-branched tubes. The sexes are 

 separate ; a single reproductive organ extends dorsally between gut 

 and aorta almost the whole length of the body ; the genital ducts are 

 paired and open posteriorly in front of the excretory apertures. The 

 ova, with chitinous spiny shells, are usually retained for some time 

 by the female between the mantle and the gills. The segmentation 

 is holoblastic, and a gastrula is formed by invagination. 



Sub-class 11. 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 flat 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 usually 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, e.g. Heteropods. There are never more than two gills 

 of the ctenidium type. The shell is usually in one piece ; but 

 It is sometimes rudimentary or absent. The foot usually 

 contains a mucus gland, and tends to be divided into three 

 regions — the pro-, meso-, and meta-podium. There is a single 

 reproductive organ and genital duct. 



Branch A. Streptoneura 



In the torsion of the body one limb of the visceral loop crosses the 

 other in a figure 8. 



Order i. Zygobranchiata 



The atrophy of the primitively left -side gills and nephridia is not 

 carried out, or only partially, e.g. Haliotis (ear-shell) ; Fissurella (key- 

 hole limpet) ; Patella (limpet). 



