SUPPLEMENT 



ON 



CRUSTACEA AND MOLLUSCA. 



NATURALISTS have experienced much difficulty 

 in finding an appropriate place for Crustacea. 

 En the present volume, they will be found 

 described along with Mollusca, in book iv. of 

 Goldsmith's history of Fishes, p. 326, et seq. 

 To Goldsmith's description we have added 

 most copious notes ; and little more will be 

 required here than to give the scientific ar- 

 rangement of the animals, as figured in the 

 illustrative plates. 



The Crustacea (which include crabs, lobsters, 

 pi awns, shrimps, &c.) rank among those ani- 

 mals which are destitute of a back bone ; and 

 are comprehended in the division termed 

 Artici lata, or animals whose members or limbs 

 consist of segments or rings, articulated into 

 each other, to the inside of which their mus- 

 cles are attached. Chemists have found the 

 shells of crustaceous animals to consist of 

 phosphate of lime in combination with animal 

 matter, while those of the testaceous mollusca 

 are composed of lime along with the gelatin- 

 ous substance. But a still more obvious dis- 

 tinction exists between these animals ; the 

 shells of oysters and other testaceous animals 

 generally consist of one or two pieces, enve- 

 loping the whole animal, (except in the Cirri- 

 pedes,seenoteon Conchology, p. 350-358) and 

 adheres permanently during life: while the 

 coverings of the Crustacea are cast and renewed 

 periodically, and invest the animals as it were 

 in a coat of mail. 



Linnasus placed Crustacea between the fishes 

 ind mollusca, united them to his class Tnsecta. 

 In the earlier writings of Cuvier and Latreille, 

 they adopted the views of Linnaeus, while 

 Lamarck followed the ideas of Brisson, who 

 formed them into a separate class, as well as 

 spiders ; giving the former the name Crustacea 

 and the latter Arachnoides. This improve- 

 ment has been generally acknowledged, and 



followed by subsequent writers on Natural 

 History. 



Crustaceous animals present remarkable 

 physiological distinctions. They respire by 

 means of branchiae, or by branchial plates, 

 usually attached to their feet or to their jaws ; 

 they have from five to seven pairs of feet; their 

 head is frequently not distinct from the trunk, 

 provided with from two to four jointed setace- 

 ous antennae ; and two compound movable 

 eyes seated on peduncles, which are some- 

 times movable, and at others fixed ; they have 

 a distinct heart, and a regular circulating 

 system : their organs of reproduction are placed 

 either in the feet or tail. 



In those genera where the head is not sep- 

 arated from the trunk, the shield or covering- 

 envelopes the whole thorax. In other genera 

 the head is distinct from the body, which is 

 divided into seven segments, to the lower sides 

 of which the feet are attached ; these for the 

 most part have a tail, consisting of many seg- 

 ments. The limbs vary from ten to fourteen, 

 each having six articulations. The two an- 

 terior limbs, and sometimes even three on each 

 side, are provided with forceps ; at other times 

 they are terminated by simple hooks, and in 

 many instances by appendages which fit them 

 for swimming. 



The branchiae in crustaceous animals, un- 

 like those of fishes, are external, although fre- 

 quently concealed, and placed at the sides of 

 the feet, limbs, or under the tail ; they are, 

 however, more frequently at the sides of the 

 feet ; and consist either of filamentary tufts, 

 or pyramidal laminae. 



The mouth has usually two mandibles, a 

 labiitm, or lip below, and from three to five 

 pairs of jaws. The first, or first three pairs 

 of these, have been termed feet-Jaws. These 

 small leg-shaped appendages are not fitted 



