HICKSON : TORSION IN MOLLUSCA. 13 



would not touch, the shell protection would be unnecessary, and any 

 increase in the size of the foot would be a positive advantage. 



What we do actually find in the group of the Opisthobranchs is 

 this, that in a series shewing a gradual diminution in the twist of the 

 shell, we find a corresponding increase in the size of the foot, both as 

 regards its mass and the surface it exposes to the ground. The in- 

 creased activity in locomotion is exhibited in the remarkable migra- 

 tions which are so characteristic of the Opisthobranchs. Their 

 distastefulness is shown in the fact that none of them are used as 

 human food, and I have heard of none that are used for bait. Prof. 

 Herdman and Mr. Cole have made a series of investigations which 

 prove that many of the Nudibranchs are really distasteful to fish. 



But the untwisting of these animals was not only due to the 

 need of increasing the foot area for purposes of locomotion, but 

 partly also to facilitate the return to a condition of bilateral symmetry. 

 All animals that move rapidly are bilaterally symmetrical. It stands 

 to reason that the form of body that is best suited to rapid movement 

 is one in which the right and left sides are evenly balanced. Fish, 

 cuttlefish, lobsters, segmented worms, and flat worms, not to mention 

 steam-engines and steam-ships, are all bilaterally symmetrical. 



There is another method by which the Gastropods may have 

 been able to dispense with the protection afforded by a capacious 

 shell, and that would be by adopting a habit of making short jumps 

 or flights through the water, after the manner of Pecten. Although 

 such habits would not protect them from fish and other free-swimming 

 enemies, it would afford them a means of escape from many of the 

 Crustacea, from predaceous Gastropods, and from other creeping and 

 crawling enemies. One of the first steps in this direction would be a 

 diminution in the weight of the shell and a lightening of the soft 

 tissues of the body by the absorption into them of a relatively large 

 quantity of sea-water. 



We see in Aplysia, one of the Tectibranchs which still crawls, 

 an example of a body that has become in this manner lighter, softer, 

 and more gelatinous in consistency. This lightening of the tissues by 

 the absorption of water is usually accompanied by an increase in their 

 transparency, and we find examples of this not only in the mollusca, 

 but in the jelly-fish whose bodies are composed of no less than 95 

 per cent, of water, in the free-swimming Tunicates, in the pelagic 

 worms, and in other groups of animals. If such a transparency of the 

 body were acquired, it would serve not only the purpose of reducing 

 the weight of the body, thus rendering possible longer and more sus- 

 tained flights through the water ; but it would also be a protection to 

 the animals in rendering them less conspicuous to their free-swimming 



