PHYLUM MOLLUSC A 193 



Physa, Planorbis, and Lymnaea probably were derived from 

 terrestrial ancestors whose descendants have again returned to 

 the aquatic habitat. 



The visceral mass hes dorsal to the foot. With increase in its 

 size, it frequently assumes a spiral form, and the mantle, which 

 is carried with it, continues to secrete a shell at the free margin. 

 As a result, the shell assumes a spiral form. The shell may be 

 either simple cone-shaped or anything between this and a highly 

 complex spiral. In the spiral type of shell, the coiled chamber 

 usually surrounds a calcareous pillar which marks the axis of the 

 shell and is termed the columella. In any given species, the 

 coiling of the shell about the columella is normally in a fixed 



Fig. 92. — Diagrams to show shifting of organs in the development of the 

 streptoneurous condition of gastropods. All diagrams as though viewed from 

 dorsal surface. A, an orthoneurous gastropod with posterior anus, and gills, 

 ganglia, and auricles arranged in perfect bilateral symmetry; B, hypothetical 

 intermediate condition in torsion; C, streptoneurous condition with gill, auricle, 

 and parietal ganglion of original left side now on right side of body. {After Lang) . 



direction. Most coiled shells when held with the apex point- 

 ing upward and the aperture facing the observer have the 

 aperture on the right side and are therefore said to be dextral. 

 Some snails are characteristically sinistral {e.g., Physa). jMuscles 

 from the body attached to the columella prevent the snail from 

 being able to come clear out of the shell. 



Since growth takes place through the addition of new shell 

 material by the mantle around the aperture, each whorl is 

 successively larger than the preceding one. When a siphon is 

 present, the aperture is drawn into an elongated process for 

 containing it. 



The nervous system consists of two cerebral ganglia, the pairs 

 of pedal and visceral ganglia, and two or three additional pairs 

 all of which are united by commissures. The cerebrovisceral 



