46 NATURAL HISTORY OF ARTHROPODS. 



place. All other Decapods remain awhile longer in the egg, and usually a free life 

 begins with a zoeal condition, which, however, is subject to various modifications, 

 which will be mentioned in their proper place. 



So far as the writer is aware, all Decapods follow two modes of casting the shell. In 

 the Macrura the cai-apax splits longitudinally down the back, and the body is withdrawn 

 through the opening thus produced. In the Brachyura, on the other hand, the splitting 

 of the shell, though stiU dorsal, is transverse, and takes place between the last segment 

 , of the thorax and the first of the abdominal series. As a preparation for casting the 

 shell Dr. Braun describes in the crayfish a series of hau'S, developed at about the time of 

 moulting, on the surface of the hypodermis, which serve to lift up the old and hardened 

 integument. Before the time of moulting there is developed on either side of the 

 stomach an oval mass of carbonate of lime, which is usually regarded as. a supply stored 

 up for the calcification of the new integument. From the fact that in the crayfish 

 these " crab-stones " weigh but about two grains. Professor Huxley is disposed to ques- 

 tion this explanation, that small amount being but slight in proportion to the animal. 

 In the lobster, and in other forms, these stones are much larger, and there the objec- 

 tion of insufficiency would hardly apply. 



Connected with the moulting is the reparation of injuries. When a crustacean 

 loses a leg, or suffers an injmy in any part, with the succeeding moults the damages 

 are repaired, but not always perfectly. Some interesting observations on this point 

 have recently been made by Dr. Faxon, especially on the claws of the lobster. Fre- 

 quently when the claw is injured, instead of acquiring its former shape, there is a ten- 

 dency toward the formation of another pincer ; the two jaws are formed, their inner 

 margins become armed with teeth, but the apparatus cannot answer for a pincer, for 

 the joint allowing it to be closed is never formed, and the two parts are never able to 

 come together. This tendency of nature to reproduce paits forms a very interesting 

 subject for investigation, for which, aside from a meaningless jingle of words, no ex- 

 planation has yet been given. 



Many Crustacea have the power of producing a noise, but whether these sounds are 

 employed as calls, the evidence as yet presented, does not enable us to decide. In 

 many forms, as Gelasimus, Ocypoda, and Pcdinurus, these sounds are produced by 

 rubbing together two parts of the hardened integument, and frequently stridulating 

 organs for this purpose are well developed. A description of that in Ocypoda will 

 suffice. In all the species of this genus there occurs in the inside of one of the large 

 pincers a row of closely set granules, which can be rubbed across a corresponding ridge 

 on the carapax, producing a noise closely resembling that which results from rubbing 

 some hard substance over a ooai-se file. In many species of the genus Alpheus, the 

 movable finger of the large claw is armed with a strong tooth, which fits into a corre- 

 sponding socket in the immovable thumb. By opening the claw and drawing the tooth 

 from the socket these small Crustacea are able to produce a noise similar to that pro- 

 duced by snapping the finger-naUs together. 



It has been stated several times in the preceding pages that the Crustacea are 

 essentially aji aquatic group, but there are some which always live on land, only re- 

 pairing to the water for the purposes of reproduction. So far have these forms become 

 accustomed to a terrestrial life and an atmospheric respiration, that Fritz Miiller has 

 proved by actual experiment that it is possible to di-own a sand crab (Ocypoda) by a 

 prolonged immersion in water, and we can do no better than to reproduce here some 

 of his remarks upon the respiration of other terrestrial Decapoda, quoting from the 



