PRtfF. AGASSIZ'S 



So that, notwithstanding the functions of these 

 parts, which are used to crush food before it passes 

 into the alimentary canal, we see that they are a 

 modification of the same appendages which on 

 the side constituted simple legs. Here, jaws and 

 legs are really modifications of one and the same 

 type of appendages. 



But the chest is not one continuous mass, (Plate 

 VI). It consists of several rings, united in the full 

 grown individuals, but distinct in the young, and 

 still distinct on the lower surface of the adult 

 These transverse rMges, (Plate XVII, fig. B) which 

 are noticed between the legs, indicate the foints, 

 which, by their re-union, constitute the chest, or 

 eephalo-thorax. And so we cannot wonder that 

 there are as many pairs of legs as there are joints 

 united to form the eephalo-thorax. These five 

 pairs of legs of the chest are figured separately, 

 (Plate XXI, figs. I, K, L, M, N). 



But, are we allowed to consider the cepbalo tho- 

 rax as consisting simply of five joints, and one for 

 the bead ? If it be true that every joint can have 

 but one pair of moveable appendages, then we 

 must admit that the head, however contracted, is 

 the result of the re-union of nine distinct joints. 

 The eyes, the palpi, the three pairs of jaws, and 

 the three pairs of jaw- feet. And indeed, so many 

 transverse divisions may be noticed in the interior 

 of the chest, in its anterior extremity, when ex- 

 amined closely ; it can scarcely be doubted, there- 

 fore that it is out of so many joints that the eepha- 

 lothorax has been formed. 



At the posterior part of the body, under the tail, 

 we have other appendages, which assume the 

 shape of branched threads, as represented in Plate 

 XXI, figs. 0. P, Q, R, S. These are modified legs, 

 which are not used in locomotion, but to which 

 the eggs become attached when they are laid, and 

 as they remain suspended to the lower side of the 

 tail, they are carried about by the female Crabs till 

 the young are hatched. The fin-like appendages 

 at the extremity of the tail, (Plate VI), are still 

 other modifications of legs ; and so, throughout the 

 longitudinal axis of such an animal, whatever 

 shape ifcs body assumes, whether in Insects or 

 Crustacea, the appendages used as legs or as jaws, 

 are only modifications of one and the same sort of 

 organs. 



It was important to come to this conclusion, in 

 order to be allowed to compare the various appen- 

 dages which were noticed on the side of many of 

 these other Crustacea, (Plate XVIII). For in- 

 stance, in Squilla, (Plate XVII, fig. D), we have a 

 kind of claw, of a very different nature. It is no 

 longer as we see it in the Crab, but it is the ter- 

 minal joint which is bent over the preceding one. 

 So that the claw here would resemble the motion 

 of my arm pressing against the shoulder, and 

 forming a forceps, not by the antagonistic action 

 of two articulations moving against each other, as 

 in the Lobster, but by the bending of the last joint 

 against the preceding one. 



Many other modifications of these appendages 

 are noticed on the sides of the body of Articulata; 

 but the time will not allow me to give all these de- 

 tails ; I merely refer to them for the sake of further 

 comparisons. Let me only show that here in Sto- 

 mapoda or Amphipoda, there is a difference of ar- 

 rangement in plate XVII, fig. I>, and plate XVIII, 

 different from what we have in Crabs and Lobsters. 

 The gills are entirely internal in Lobsters and Crabs; 

 in the Squilla they are below the rings. Is there an 

 essential difference in such a position ? No, there 

 is not. If we look at the embryo Crawfish,as it has 

 been figured by Eathke, we shall know that the 

 shield, or the external covering, is gradually modi- 

 fied by the development of the shield, which grows 

 successively over the gills. The gills are external 

 where they are attached to the lower joints of the 

 legs, and are not different in their nature, but only 

 modifications of one and the same type. 



All the Crustacea belonging to these two groups i 

 or rather to these three groups the Crabs, the Lob" 

 sters, and the Squilla are among the larger of the 

 class. The other types, represented (Plates XVIII, 

 XIX, and XX) are almost universally small some 

 even microscopic. In the Amphipoda (Plate 

 [PLATE XVTTI Low SPECIES OF CRUSTACEA.] 



