550 THE INVERTEBRATA 



ance. Besides the primitive character found in the chitons they have 

 others, one of which is the free communication of the parts of the 

 coelom. The gonads open into the pericardium and a pair of coelomo- 

 ducts (probably corresponding to the kidneys) convey the gametes 

 from the pericardium to the exterior. The radula varies greatly, all 

 stages from absence to a type with several transverse rows of teeth 

 being found. This condition may also be considered as primitive. 



Class GASTEROPODA 



Mollusca with a distinct head bearing tentacles and eyes, a flattened 

 foot, and a visceral hump which exhibits the phenomenon of torsion 

 in various degrees and is often coiled; always exhibiting bilateral 

 asymmetry to a certain extent; typically with a shell secreted in a 

 single piece; nervous system with cerebral, pleural, visceral and 

 usually pedal ganglia and a visceral loop ; a radula ; often a trocho- 

 sphere larva. 



We can safely say that the Gasteropoda are descended from sym- 

 metrical unsegmented ancestors (p. 543), and that the most prominent 

 differences among their present-day representatives are due to the 

 varying degrees in which they exhibit the phenomena of torsion. The 

 ancestors of the Gasteropoda had not been affected by torsion. They 

 possessed a symmetrical body with a straight alimentary 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 



