IX 



MOLLUSCA 



317 



shell may never be spirally twisted at all, as in Acmaea, and 

 presumably in Patella, and yet the torsion may reach its extreme 

 limit ; moreover, torsion is always complete before the spiral twisting 

 of the shell begins, as is well seen in the development of Paludina. 

 Both changes are due to the unequal growth of the two sides of the 

 animal ; but whereas torsion affects the whole area of the side of the 

 body above the insertion of the foot, the inequality of growth resulting 



Fig. 251. — Just liatclied Paludina. vivipara. Seen from the left side and viewed as a 

 transparent object. (After Erlanger.) 



a.p, anal papilla ; b.g, buccal ganglion ; br, rudiments of the gill ; eg, cerebral ganglion ; gon, 

 rudiment of genital organ ; H, heart ; liep, liver ; int, intestine ; l.ur, left ureter ; m.f, mantle fold ; 

 oc, eye; op, operculum; osph, osphradiura ; ot, otocyst; ped, pedal nervous cords; per, pericardium; 

 r.k, right kidney ; r,s, radula sac ; r.ur, right ureter ; sal, salivary gland ; st, stomach ; v.l, visceral loop 

 of the nervous sy.stem. 



in spiral twisting of the shell affects only a more dorsal region, 

 leaving the floor of the mantle cavity unaffected. It is accompanied 

 by a lengthening of the visceral hump — and this is associated by Miss 

 Drummond (1902) with the outgrowth of the embryonic stomach so 

 as to form the adult liver. The most plausible explanation of the 

 inequality of growth, in both torsion and twisting, is that in the 

 ancestral gastropod the lengthened visceral hump fell over to one 

 side as the animal took to crawling over uneven ground. As a 



