52 



NATURAL HISTORY OF ARTHROPODS. 



FIG. 62. Alpheus heterocheles, twice natural size. 



minated with pincers, one being small while the other is enormously developed, being 

 as large as the cephalothorax. 

 This genus is almost exclus- 

 ively marine, but some species 

 are occasionally found in fresh 

 water as well. In one of 

 these from Florida (Alpheus 

 minus) the marine forms are 

 very small, while specimens 

 obtained from fresh water, 

 belonging to the same species, 

 were nearly three times as 

 large. 



None of the Caridea are 

 true parasites, though a few 

 are commensals, that is, they 

 are closely associated with 

 other animals. Thus some species of Alpheus and Pontonia live within the shells 

 of certain molluscs. 



The ASTACOIDEA is a much more important group than the one that we have 

 just left, embracing as it does many large species possessing an alimentary interest. 

 Without entering into the characters limiting this group, we may proceed to divide it 

 into three families, Astacida?, Loricata, and Thalassinida'. 



The ASTACID^E, in their shape show a close approximation to the Caridea, but are 

 distinguished from them by having the epistome united to the carapax, as it is in all 

 the higher forms, while on the dorsal surface of the carapax is a transverse suture 

 (wanting in the Caridea), which, as we have seen, is the remains of the joint between 

 the antennal and mandibular somites. All three (and in the Eryoninas four or five) 

 of the anterior pairs of thoracic feet are terminated by a pincer, the first pair being very 

 large, and forming the well-known " claw " of the lobster. 



The Astacidae, so far as is known, differ from the rest of the Decapods in leaving 

 the egg in nearly the adult condition, the zoeal stage being suppressed, the youngest 

 larva being in the Mysis stage in the case of the lobster, while in the fresh-water cray- 

 fish the young differs in only unimportant details from the adult. The genera, of 

 which about fifteen have been described, are distributed about equally between marine 

 and fresh-water forms, and may be divided into two sub-families, the Eryoninae and 

 Astacinaa. The former, as has been stated, being characterized by four or five pairs of 

 chelate feet, the latter by three. The Eryoninae were long considered as an entirely 

 extinct group, but recent deep-sea dredgings have brought to light several forms which 

 have been described under the generic names of Polycheles, Pentacheles, and Witte- 

 moesia. The genus Eryon occurs fossil in the Solenhofen lithographic stone (Upper 

 Oolite). The sub-family is exclusively marine. 



The Astacinas contains the crayfish and lobsters, or fresh and salt-water forms. 

 Though several genera have been described, only Cambarus, Astacus, and Homarus, 

 need here be mentioned. Cambarus and Astcwus, our types of crayfish, differ from 

 each other in only unimportant details ; but the distribution of our American species 

 presents an interesting feature. The genus Astacus occurs on the Pacific slope 

 (and in Europe as well), while the waters which flow into the Atlantic contain only 



