MOLLUSCA 551 



Other, the whole organ is twisted into a compact spiral which can be 

 arranged so as not to interfere with the balance of the animal while 

 crawling. 



In all gasteropods with coiled shells the mantle cavity is anterior, 

 the opening directed forward and the coiling of the visceral hump is 

 directed posteriorly. But in the development of these forms from the 

 larva (Fig. 378) the mantle cavity first makes its appearance behind 

 the visceral hump, and at a particular stage the visceral hump rotates 

 in a counter-clockwise direction through an angle of 180° on the rest 

 of the body (Fig. 378 D). This is what is known as torsion, and as 

 shown above it is entirely distinct from the coiling of the visceral 

 hump which precedes it, though it may have been necessitated by the 



Fig. 378. To show torsion in Paludina vivipara. After Naef. Embryos seen 

 from the side (A, B, D) and behind (C). A, Almost symmetrical stage, with 

 mantle cavity behind but anus twisted a little to the right. B, Stage showing 

 90° of torsion, mantle cavity and anus to the right. C, Torsion at almost the 

 same stage as B. D, Stage showing i8o° of torsion and the adult condition. 

 an. anus; m.c. mantle cavity; op. operculum; vm. velum. 



antecedent phenomenon. Only the narrow neck of tissue (and the 

 organs which pass through it), between the visceral hump and the 

 rest of the body, is actually twisted ; but the orientation of the mantle 

 cavity and its organs is changed (Fig. 379). Before torsion the 

 ctenidia and the anus point backwards, the auricles are behind the 

 ventricle. After torsion the ctenidia project forward, the auricles are 

 in front of the ventricle; the mantle cavity opens just behind the head. 

 The uncoiled visceral loop has been caught in the twisting and one 

 connective laid over the other, one passing over the intestine and the 

 other underneath, but both coming together near the anus and com- 

 pleting a figure of eight. The whole process takes only two or three 



