GASTEROPODA 477 



possessed a symmetrical body with a straight aUmentary canal ending 

 in a posterior anus. On each side of this was a ctenidium, that is, a 

 breathing organ composed of an axis with a row of leaf-like branches 

 on each side. The ctenidia may have been free on the surface when 

 they first arose, but they were soon contained in the posterior mantle 

 cavity which developed with the visceral hump. 



Many characters belonging to the primitive mollusc are still pre- 

 served in the gasteropods, the head with tentacles, the nervous system 

 with cerebral, pleural, and pedal ganglia, the radula, the ventricle 

 with two auricles and the two kidneys. Lastly, there is a flat creeping 

 foot and a visceral loop formed by a connective from each pleural 

 ganglion uniting with its fellow in the neighbourhood of the ctenidia. 

 In the alimentary canal of molluscs there is a tendency for digestion 

 and resorption to be confined to a dorsal diverticulum of the alimen- 

 tary canal which develops into the digestive gland (liver). The growth 

 of this causes the formation of a projection, the visceral hump, and a 

 looping of the alimentary canal. This projection grows until it falls 

 over, and this is the first step in the coiling of the visceral hump which 

 is such a characteristic feature of the gasteropods. Growth proceeds 

 until, in the snail, for instance, the visceral hump would, if uncoiled, 

 be longer than the whole of the body. Owing, however, to the fact 

 that one side of the hump grows faster during development than the 

 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. 354) 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. 354 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 

 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. 355). 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 



