314 MOLLUSCS. 



features with Chiton, but much simpler are Neomenia, Promomenia, and 

 Chcetoderma, in which the shell is represented only by small calcareous 

 plates and spines in the skin. The three genera last mentioned suggest 

 ideas about a hypothetical worm-like ancestor of Gasteropods. 



B. Asymmetrical Gasteropods (Anisopleara). — Excepting Chiton and 

 its allies, the Gasteropods are more or less asymmetrical. We must 

 notice, however, (l) that this want of symmetry does not affect the head 

 or the foot, but only the visceral mass of the body which is more or less 

 twisted round to the right side toward the head ; (2) this torsion must be 

 distinguished from the frequent spiral twisting of the visceral hump and 

 of the shell ; (3) the torsion occurs in variable degree, and some forms, 

 especially free-swimmers, have a superficial symmetry. 



Professor Ray Lankester suggests an explanation of the asymmetrical 

 torsion, which is none the less welcome that it does not seern to be 

 quite consistent with his opposition to Lamarckian theory, or in other 

 words, to the explanation of peculiarities of structure as the results 

 of use, disuse, and conditions of life. I shall quote a few sentences 

 from his highly appreciated article on MoUusca in the Encyclopedia 

 Britannica : — 



" This torsion is connected mechanically with the excessive vertical 

 growth of the visceral hump, and the development upon its surface of a 



heavy shell Whilst such a shell might retain its median position 



in a swimming animal, it and the visceral hump necessarily fall to one 

 side in a creeping animal which carries them uppermost. The shell 

 and visceral hump in the Anisopleura incline normally to the right side 

 of the animal. As mechanical results, there arise a one-sided pressure 

 and a one-sided strain, together with a one-sided development of the 

 muscular masses which are related to the shell and foot. Both the 

 torsion through a semicircle of the base of the visceral dome, and the 

 continued spiral growth of the visceral dome itself, which is very usual 

 in the Anisopleura, appear to be traceable to these mechanical conditions. " 



The above explanation seems very reasonable, but however the lop- 

 sidedness of most Gasteropods has arisen, its existence must be realised 

 if the other anatomical characters are to be understood. For the 

 torsion has numerous more or less direct results. Thus (a) the food- 

 canal ends on the right side, usually far forward on the neck ; {b) the 

 primitively left gill, brought round to the right by torsion, usually 

 atrophies ; (c) the primitively right kidney seems to disappear, while the 

 primitively left, brought round to the right by torsion, persists ; at any 

 rate there is only one ; (d) in one series of Gasteropods the visceral 

 nerve-loop, running from the cerebral and pleural to the visceral 

 ganglia, is " caught in the twist " and twines like a figure 8 (Strepto- 

 neural condition), in the others the same visceral loop is short and 

 untwisted (Euthyneural condition). 



To sum up, excepting a small sub-class of symmetrical 

 forms (tf.^., Chiton), Gasteropods are asymmetrical molluscs, 

 in which the visceral mass has undergone more or less 

 forward torsion toward the right side. This torsion pro- 

 foundly affects the food-canal, gills, kidneys, and often the 



