6630 



Crustacea. 



The relation of one of these to Balanus balanoides, with which it is 

 found associated, was mistaken by Mr. Goodsir for the male Cirripodia, 

 but which Mr. Darwin has shown, from the hermaphrodite character 

 of the latter, must be a parasite inhabiting the shell. The females 

 of these genera appear to be quite blind. 



There are parasites among Crustacea the eyes of which appear to 

 be monstrously developed, as in some of the Hyperidae ; but this is 

 no evasion of the general scheme, that the organs of vision shall be 

 in accordance to the wants of the animals. Hyperia Galba and 

 many others take up their abodes within the transparent Medusae, and 

 swim the clear surface of the broad ocean, floating in their balloon- 

 like cars. These, though parasites, live in the clear effulgence of the 

 day, but which must be lessened in degree, independent of the 

 passage of its beams through the water, by the interference of the 

 tissues of the animal, in the hollow chambers of which they reside. 

 The eyes of Hyperia are large, and embrace in compass nearly the 

 whole of the head of the animal ; the colour is of a pale and delicate 

 green. But under whatever colour or circumstances perceptible, the 

 organ seems developed in a proportion best suitable to the wants of 

 the particular being it gives light. 



When the range of vision is required to extend far, the eye is 

 carried upon foot-stalks, and these, in some species, are not much 

 less important than the legs of the same animal, as in the exotic 

 genus Podophalmata, and in the beautiful little translucent stomapod, 

 the Philosoma of the temperate ocean. 



Sometimes they are lodged beneath the integuments ; and, again, 

 they are brought so near together that they converge into a single 

 organ ; and in some they appear to be wanting, though this is pro- 

 bably owing to the absence of a coloured cornea. But, wherever the 

 eye is placed or formed, it holds, homologically, the same position 

 throughout Crustacea, and must be looked upon as first of a series of 

 members formed to suit peculiar conditions ; one is developed to act 

 as a leg; another to assist in swimming; a third is formed so as to be 

 useful as a mandible. Under all these various changes the appendages 

 are to be seen, each one adapted for an especial use ; thus, the eyes 

 are but tactile organs, modified so that the nerves may receive im- 

 pressions of light, just in the same way as we may suppose an 

 extended arm would hold a ball, or, to speak more correctly, the 

 hand itself be converted into an eye-ball. 



(To be continued.) 



