;326 On Light thrown on Geology by Submarine Besearches, 



very little power of progressing to any distance, when fully 

 developed. The " shifting" or migration is accomplished by 

 the young animals when in a larva state. This brings me to 

 A second point, which needs explanation. All mollusca 

 undergo a metamorphosis either in the Qgg^ or out of the Qgg^ 

 but, for the most part, among the marine species out of the 

 e%g. The relations of the metamorphoses of the several 

 tribes are not yet fully made out ; but sufficient is now known 

 to warrant the generalization. In one great class of mollusca, 

 the Gasteropoda, all appear to commence life under the same 

 form, both of shell and animal, viz. a very simple, spiral, 

 helicoid shell, and an animal furnished with two ciliated wings 

 or lobes, by which it can swim freely through the fluid in 

 which it is contained. At this stage of the animaVs existence, 

 it is in a state corresponding to the permanent state of a Ptero- 

 pod* and the form is alike whether it be afterwards a shelled 

 or shell-less species. (This the observations of Dalyell, Sars, 

 Alder and Hancock, Allman, and others prove, and I have 

 seen it myself.) It is in this form that most species migrate, 

 swimming with ease through the sea. Part of the journey 

 may be performed sometimes by the strings of eggs which fill 

 the sea at certain seasons, and are wafted by currents. My 

 friend, Lieut. Spratt, R.N., has lately forwarded me a drawing 

 of a chain of eggs of mollusca, taken eighty miles from shore, 

 and which, on being hatched, produced shelled larvae of the 

 forms which I have described. If they reach the region and 

 ground, of which the perfect animal is a member, then they 

 develope and flourish ; but if the period of their develop- 

 ment arrives before they have reached their destination, they 

 perish, and their fragile shells sink into the depths of the sea. 

 Millions and millions must thus perish, and every handful of 

 the fine mud brought up from the eighth zone of depth in the 

 Mediterranean, is literally filled with hundreds of these cu- 

 rious exuviae of the larvae of moUusca.f 



* It is not improbable that the form of the larva of the Pteropod, 

 when it shall be known, will be found to be that of an Ascidian polype, 

 even as the larva of the Tunicata presents us with the representation of 

 a hydroid polype. 



t The nucleus of the shells of the Cephalopoda is a spiral-univalve 



