6628 Crustacea. 



with four eyes. Without doubt there are four distinct lenses, two to 

 each eye ; and these are placed at the extremity of each organ, and 

 implanted in the integumentary structure ; from which we may 

 assume that an arrest in the number of lenses takes place at an early 

 stage, and that in the Ampelisca they do not increase in number, as 

 is the case in the Amphipoda generally. 



The distinction of the character of the eye in the Amphipoda and 

 Isopoda from that of the Decapoda appears, in an anatomical point of 

 view, to rest chiefly upon the peduncle being absent in the former, 

 rather than to any definite alteration in the eye itself; and by no 

 means is it the test of any inferiority on the part of the animal. This 

 appears to be easily demonstrated by the fact that in all the Diastylidae 

 the eyes are sessile, and converge into a single organ ; this is the case 

 also with some of the Entomostraca ; but, on the other hand, the genera 

 Tanais, among the Isopods, and Nebalia and Artemia, among Ento- 

 mostraca, have the eyes supported upon foot-stalks, in a manner cor- 

 responding with the decapod Crustacea. 



The eye appears to be designed to suit the want of the animal, and 

 is developed according to the habits of the creature, being more im- 

 portant in those which are erratic and swim about in the light, as 

 Nebalia and the Macroura. Take a prawn for instance, and see, in 

 the vivarium, how it will gracefully swim about in search of food ; and 

 if you are in the habit of feeding it, as one which. I recently noticed 

 in the possession of Dr. Dansey, it soon becomes very tame, and 

 knows the stick that hands its food to it, which as soon as it perceives 

 comes over to it. But to those animals which are so situated that 

 eyes can be of no use, as the fossorial Crustacea, whose residence is 

 in the dark; or, again, as to the barnacle tribe, which reside in the 

 light, perfect vision would have been an evil ; for animals stationary 

 as they are could not have procured food, let them be ever so hungry, 

 that they saw swimming at a distance : sight is granted them only to 

 avert danger. When lashing the water with their plumose cirri, they 

 know no danger until an ol ject swims between them and the light, 

 when they immediately dart in and close their valves. Diogenes 

 waits until Alexander has passed by. 



It is probable that some may believe, and it was a fancy of my 

 own for a time, that the power of sight in the inhabitants of the water 

 must be very imperfect, and probably indistinct in outline. I 

 recollect how indistinctly objects are seen when diving under water, 

 but we also know how considerably objects are magnified when 

 seen through the water, and that a small alteration of the convexity 



