52 



NATURAL HISTORY OF ARTHROPODS. 



Fig. 62. — Alpheus heterockeles, 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 fonns 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 



ti-ue parasites, though a few 



are commensals, that is, they 



are closely associated with 



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



of certain molluscs. 



The ASTACOIDEA is a muclumore 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, — Astacidae, Loricata, and Thalassinidse. 



The AsTACiD^, 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 Eryoninse 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 Astacidse, 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 suppi'essed, 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 Eryoninse and 

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

 chelate feet, the latter by three. The EryoninsB 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, Pentachdes, and WiUe- 

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

 Oolite). The sub-family is exclusively marine. 



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

 Though several genera have been described, only Cambarus, Astacics, and JTomarus, 

 need here be mentioned. Cambarus and Astacus, 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 Astacics occurs on the Pacific slope 

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



