Crustacea. 



6627 



of parts, and as an animal has itself passed out of the list of the 

 living. 



Among mammals we find the organs of vision adapted to the pecu- 

 liar wants of the animal. The mole, which burrows where but little 

 light can reach, possesses a very small eye. This regulation is also 

 carried out among animals of the lower order. Some of the shrimps 

 are known to live deep in the mud and sand, and some amphipods 

 dwell in dark underground wells. The Thalianassa which was found 

 by Colonel Montagu at the mouth of the Kingbridge river inhabits 

 passages in the sand at the depth of a foot or more ; the Gebia, also 

 a closely-allied genus to the last, dwells in the mud;*and in Plymouth 

 Sound, Dr. Leach has excavated individuals, and found their sub- 

 terranean passages to extend in a horizontal direction often to the 

 length of a hundred feet or more ; the Callocaris Macandrise is stated 

 by Mr. Bell to be fossorial at the depth of eighty fathoms. Positions 

 such as these, we should suppose, must render the eyes useless as 

 organs of vision ; we therefore find that in these and similar-dwelling 

 Crustacea the eyes are reduced to a rudimentary condition ; and to 

 such a degree in nature has this fact become constant that we are 

 fully justified, upon finding an animal the habits of which are 

 unknown, with but imperfectly-developed organs of vision, to infer 

 that they are generally so placed as not to require their use. An 

 instance may be given in one of these forms that is figured in Dana's 

 great work on Crustacea (Cryphiops spinuloso-manus), where the 

 eyes are placed so far back beneath the carapax that they cannot be 

 seen until a considerable portion of the latter be removed. We can 

 scarcely doubt that in its natural state this animal is subterranean in 

 its habits. 



Ampelisca Gaimardii, Kroyer. 



In the Amphipoda, nearly the whole of which reside beneath stones, 

 or in dark recesses at the bottom of the sea, the eyes are lodged 

 beneath the common integument. Only one genus is an exception to 

 this rule, and that is the remarkable Ampelisca, an animal apparently 



