812 NATUEAL HISTOET OP AQUATIC uilSriMALS. 



chemical changes in the organic tissues, when life is no longer present to resist the ordained 

 agency of decay and decomposition; in fact, it is a slow combustion by combination with oxygen. 

 " The presence of phosphorus in the Lobster is of great importance to the consumers of these 

 sea luxuries; there is no substance which conveys phosphorus so readily into the human system 

 in an agreeable form, and which the system so readily and quickly assimilates, as the flesh of 

 Crabs and Lobsters." 



225. THE CRAY-FISHES— ASTACUS AND CAMBARUS. 



Eelations and distribution. — The so-called "Cray-flshes'ior "Craw-fishes" are common 

 inhabitants of most of the fresh- water streams of the United States and Europe, but in this 

 country they are not eaten nearly to the same extent as in some parts of the Old World,-.and 

 they are not generally regarded here as a staple article of food. 



The l«rorth American Cray-fishes, although belonging to but a single family, the Astacidce, 

 constitute two distinct genera, Astacus and Gambarus, and about thirty-eight species, three of 

 which do not, however, occur within the limits of the United States. The Cray -fishes bear a strik- 

 ing resemblance to the Lobsters {Homa'rus), to which they are closely related, but there are 

 several important structural differences between them, and none of the Cray-fishes grow nearly 

 as large as the Lobster. 



Europe contains only three species of Cray-fishes, all belonging to the well-known genus 

 Astacus. They are A. nohilis Huxley {fluviatilis), A. torrentium, and A, leptodactyltis. Much 

 discussion has taken place avofing naturalists as to whether the above species are really distinct 

 from one another, or merely form varieties of a single variable species. The relations of ^. nobilis 

 to A. torrentium are more marked than of those two forms to A. leptodaetyhis, but probably the 

 specific differences pointed out are as good as exist between many other unquestioned species of 

 the same group in this country. It matters little to us in this connection, however, what mg,y be 

 the true affinities of these forms to one another, as long as we can define them sufficiently well to 

 speak of their relations to the fresh-water fisheries of Europe, as an introduction to our own species. 



Asta(m,s nobilis and A. torrentium are the edible Cray-fishes of Western Europe, and inhabit 

 fresh- water streams generally. They "are intermixed over a large part of Central Europe. 

 A, torrentium has a wider northwestward, southwestward, and southeastward extension, being 

 the sole occupant of Britain, and apparently of Spain and of Greece. On the other hand, in the 

 northern and eastern parts of Central Europe, A. nobilis appears to exist alone. Farther to the 

 east a new form, A. leptodaetylus, makes its appearance." > Those who have treated of the two 

 western species of Europe from a practical standpoint have generally spoken of them as a single 

 species, to which the old and well-known name of fluviatilis has been applied. This distinction 

 corresponds with the views of the older European naturalists, and this species thus, constituted, 

 with its two or more varieties, is the common fresh-water Cray-fish of European literature, which 

 has come to have a world-wide reputation. 



The structure of the Cray-fish can be best described by defining some of the principal 

 characters in which it differs from the common Lobster, according to Huxley. The generarshape 

 of the body with its appendages and the general make-up of the two forms are very similar; 

 but the Cray-fish has only eighteen pairs of perfect gills or branchiae at the most, and the Lobster 

 twenty. "Moreover, the branchial filaments of these giUs are much stiffer and more closely set" 

 in the Lobster than in most Cray-fishes. The most important distinction, however, is presented 

 by those gills which are attached to the bases of the thoracic limbs, and which number six pairs 



'Huxley : The Cray-fish, 1880, pp. 299, 300. 



