420 



CRUSTACEA. 



claws are nearly equal, consisting also of a single species (Arius stirln/nchus, Leach) found upon the coasts of 

 England and France. 



Amongst those species which have the six fore-legs forming as many didactyle claws — (a character which removes 

 them from all the preceding Decapods, and in which they are related to the species at the head of the following 

 section— from which, however, they differ in the fore-claws being by far the largest, the peduncle of the lateral 

 antenna; furnished with a scale or spines, the outer plate of the swimmeret at the extremity of the tail appearing 

 in all the recent species, as though it is divided into two parts by a transverse suture,)— are the following genera. 

 Eryon, Desm., comprises a single singular fossil species found in the calcareous stone used for lithography at 

 Pappenheim and Aichtedt, in Anspach. The carapax is [very broad], and with very deep lateral incisions. The 

 plates of the swimmeret are pointed at the tip. 



The genus Astacus, Gronovius, Fabr., have the lateral plates of the swimmeret broad and rounded at the ex- 

 tremity ; the two exterior ones with a transverse suture. The two filaments of the intermediate antennae are 

 longer than their peduncles, with the sides of the carapax entire. 



In the marine species of this genus, the middle plate of the tail does not exhibit a transverse suture. Of some 

 of these, Leach has formed his genus Nephrops, characterized by the large scale of the lateral antenna;, and the 

 long prismatic claws of the fore-legs. Type, Cancer norvegicus, Linn., a species found on our coast. The 

 others having the lateral antennae only furnished with two short teeth or spines, and the fore-claws large and oval, 

 form the restricted genus Astacus, Leach, the type of which is the common Lobster (Cancer gammarus, Linn.; 

 Astacus marinut, Fabr.), of which the rostrum in front of the carapax is armed with three teeth on each side, and 

 a double tooth at the base ; and the claws are very large, and unequal in size. The flesh is highly relished. It is 

 found in the F.uropean Ocean, the Mediterranean, and on the coasts of North America. The internal structure 

 has been studied with great diligence by MM. V. Audouin and M. Edwards. 



In the fresh-water species of this genus, the terminal segment of the tail, forming the middle plate of the swim- 

 meret, is transversely divided by a suture* ; and the claws are rough, and finely toothed on the inside of the 

 fingers. The rostrum has a tooth on each side, and two at the base. It is ordinarily of a greenish-brown colour, 

 [but, like the lobster, changes to bright red by boiling]. From its common occurrence it has been greatly studied, 



not only as regards its anatomy, but also its habits, 

 and the peculiar power it possesses of renewing its 

 antennae and legs when thrown oft' or mutilated. 

 The stomach contains, at the time of moulting, two 

 stony secretions, formerly used in medicine as ab- 

 sorbents, but which are now replaced by carbonate 

 (if magnesia. It hides itself under stones and in 

 burrows [in the banks of rivulets and streams], 

 whence it only comes forth in order to search for 

 its food, which consists of small mollusca, small 

 fishes, and the larva; of aquatic insects. It also 

 feeds upon decaying flesh, and the carcases of ani- 

 mals floating in the water ; and which is also used 

 as a bait, being placed in the middle of a bundle of faggots, or in a net. Its moulting takes place at the end of 

 the spring. Two months after coupling, the female lays her eggs, which are at first collected in a maps, and 

 attached, by means of a viscid liquor, to the subabdominal false legs. They are of a bright red colour, and in- 

 crease in size before they are hatched. The Crayfish are at their birth very soft, and completely resemble their 

 parent. They take refuge beneath her tail, where they remain several days until the different parts of their bodies 

 have acquired a sufficient strength. They live to the age of twenty years, increasing in size in proportion to their 

 age. Those are preferred which are found in running water. A singular Annelidous parasite (Branchiobdella, 

 Odier, in Mem. Sue. d'llist. Nat., Paris, p. 69), first observed by Rosel, infests the branchia: of the Crayfish. 



Another species inhabits the fresh water of North America; and a third, according to Le Conte, does much in- 

 jury to the rice plantations of the same country .f 



The fourth section, Carides (Latr.), have the intermediate antennae inserted higher than the lateral, 

 and the peduncle of the latter is covered by a large scale. The body is arched, as though hunch- 

 backed, and of a more slender consistence than in the preceding Crustacea. The front of the carapax 



Pg. 6.— The Crayfish. 



* [Milne Edwards, from having adopted an evidently improper mode 

 of nomenclature, lias taken away from the two best known Decapod 

 Crustacea, the old generic natnea which they are clearly entitled to 

 retain. Thus he calls the common Crab, which is the true type of 

 the genus Cancer, Platycarciuus; against which impropriety Mr. Bell 

 has well remarked, that by any other term than Cancer to this genus, 

 we are obliged to restrict the word Cancer to a small and compara- 

 tively unimportant group, not a single species of which was probably 

 distinctly known to any naturalist of early times. In like manner, he 

 has taken away the name Astacus from the Lobster, and given it to 

 the Cravfish, and proposed the new name Homarus for the former: 

 thus doing injustice to Dr. Leach, who, in the manuscripts 

 quoted in the Entomologist's Compendium (with which Milne 

 Edwards is evidently unacquainted), had called the Lobster Astacus 

 gammarus, and the Crayfish Potamobius Jiuviutilil. It is proper. 



however, to observe, that the latter had been named Cancer astacus 

 by Linnaeus.] 



t [The developement of the embryo Crayfish, in he e^, has been 

 investigated by Dr. Hathke, in a most elaborate and satisfactory man- 

 ner, in his Untersuchungen uber die Bildung und Erttwickelung dcr 

 Flusskrebsen, fol. Leipz. 1829. Some idea of the extent of the re- 

 searches of this author upon the subject may be entertained from the 

 fact that five large folio plates are completely filled with details of the 

 structure, internal and external, of the ova, in various states of de- 

 velopement, anu of the newly-hatched animal, from whence it is im- 

 possible to arrive at any other conclusion than that the Crayfish does 

 not undergo any change of form which can in the least degree merit 

 the name of metamorphosis. A full abstract of this valuable memoir 

 is inserted in No. 18 of the Zoological Journal, and in the Annales 

 des Sciences Naturelles for August, 1831.] 



