116 THE PTEROPODS. 



the surface is reached. Light, therefore, and not the search 

 after food, or the desire of breathing a freer atmosphere, as 

 Rang believes, is, according to D'Orbigny, the true regulator 

 of their diurnal movements. It is not a sufficient objection 

 to this theory to remind me that the Pteropods are eyeless 

 and blind, for numerous facts prove that many animals 

 which have no organs of vision are still powerfully affected 

 by the light, seeking or shunning it as their sensations teach 

 them to find pleasure or pain under its influence.* But still 

 we may ask, why do the Pteropods disappear, as the darkness 

 thickens, to a retreat certainly still darker, and why are they 

 not ready to greet the dawn in its approach as well as the 

 evening fall, seeing that there must be, in both, periods at 

 which the shade of light will be of equal intensity ? Let it 

 also be remarked, that their congregations on the surface 

 are variable and inconstant. For some successive nights a 

 species will throng the naturalist's fatal net, when, without 

 any visible cause, it may be cast and drawn for two or three 

 nights in succession and in vain not a single individual has 

 left his subaqueous haunt, after which they will again 

 suddenly rise as numerous as before. It is neither an 

 instinctive prescience of a storm, nor a storm itself, which 

 hinders them, for D'Orbigny often took them during stormy 

 nights in abundance ; and the belief that at such seasons 

 they lay sunk in the abyss seems to have originated in its 

 seeming reasonableness to the naturalist, who deemed it 

 consequently fruitless to endeavour the capture of such 

 fragile creatures among the billows of a troubled sea. 



By what mechanism the Pteropods balance themselves in 

 the deep and vary their position, is, perhaps, scarcely deter- 

 mined. Cuvier conjectures, with reference to the Clio, that 

 there may be a collection of fluid or air in a space between 

 the sac and the viscera, by compressing which the animal 

 will sink ; and it will rise when the air or fluid is allowed to 

 distend the relaxed sac to its full capacity.")* D'Orbigny 

 seems to think that nothing is required beyond their 

 muscular efforts and the movements of their fins, and though 

 this explanation would seem to require a continuance of 

 action, to which there is no relaxation or end, it is better 

 to acquiesce in it than to call imaginary structures to our 

 aid. The Pteropods, says D'Orbigny, have a peculiar mode 

 of swimming in subservience to their form : the cephalic fins 

 can propel the animal forwards, or sustain it afloat, only by 



* The Clio, it is now ascertained, has two eyes, apparently of a very 

 complete character. See JONES'S Animal Kingdom, p. 428. 

 f Mem. sur les Mollusq. ii. 6. 



