?ROF. AGASSIZ'S 



species consist of individuals of two kinds ; and to 

 know those two kinds, is not sufficient to form a 

 toTect idea of the species. There are species in 

 which individuals of various kinds are combined 

 together, and in which the combination, in pro- 

 portion to the numbers which are consJant, con- 

 stitute an additional character of the species. And 

 for those, we must enlarge our notion of specific 

 limits, and introduce elements which are generally 

 overlooked. 



But I proceed to the illustration of the class of 

 Crustacea. These animals constitute, as they are 

 now circumscribed, a very natural group ^ though 

 it may be very difficult to assign general charac- 

 ters to it. And, indeed, on trying to find a practi- 

 cal traJt of character, a combination of structural 

 peculiarities, which should exclude any other an- 

 imal, and combine together all the Crustacea, I 

 have strongly felt that these animals were now 

 combined as they are, not from any anatomical 

 svidence, but from the very reason on which I 

 insist as the foundation of classification ; namely, 

 from various hints about the growth, the mode of 

 formation, and the transformations of their species. 



They are so heterogeneous in their external as- 

 pect, as scarcely to indicate animals belonging to 

 one class. Who would suppose such a congrega 

 tion of large shells, (here the Professor exhibited 

 a large bunch of Barnacles,) to be Crustacean 

 to have an animal allied, for instance, to the Horse- 

 Shoe, or to the Crabs, Lobsters, Shrimpa, and the 

 like. Nevertheless, it is certain, from what we 

 know of the metamorphoses of the Barnacles, that 

 they, too, as well as many Worm-like Parasites, 

 belong to the Crustacea. 



The importance of Embryologies! studies, for a 

 correct understanding of the true character and 

 lassification of animals, is so plain and so ob- 

 vious in the class of Crustacea, that I beg to be al- 

 lowed to illustrate more extensively this class, m 

 this respect, than I would otherwise. 



I would begin this, by pointing out some pecul- 

 iarities in their form, which have reference to the 

 changes which these animals undergo during their 

 metamorphoses. Plate XVII represents various 

 animals, all of which belong to the class of Crus- 

 tacea, 



In Plate XVII, ftg. A, is a Crab (Lupa dicantha) 

 seen from above ; and in Fig. B the same as seen 

 from below. You may notice the number of legs. 

 They are in pairs the anterior pair of which con- 

 stitute powerful claws ; the others being termi- 

 nated by a simple joint at the erd. The body is 

 so contracted that the longitudinal diameter is 

 shorter than the transverse diameter. It is a pe- 

 culiarity of almost all Crabs, that their longitudi- 

 nal diameter, if not shorter, is scarcely longer than 

 the transverse. 



Another peculiarity is, that the tail itself is 

 short and bent under the main part of the body. 

 ^Plate XVII, fig. B). 



The main body, which is neither a head nor a 



,4TK XVfT f!R4T?J A ten 



chest, but which is simultaneously both, and on 

 that account is called cephalo-thorax the head- 

 chest contains the main mass of organs ; the njrr. 

 yoas system, the alimentary canal, and the heart 

 as well as tbe respiratory organs, which are in 

 these animals attached to the legs. This peculiar,, 

 contracted form v/ill presently be found to have 

 reference to some changes,which are noticed in the 

 growth aad metamorphoses of Crustacea; aad- are- 

 therefore essential; 



On the anterior portion of the body, there are 

 thread-like appendages, called antennss or palpi 

 Of those r there are two pairs ; one an inner pai-r,. 

 and the other an external pair? and sideways from 

 those, are eyes. They are, iu these' Crab? f (Plate 

 XVII, figs. A, B) placed in a little depression on 

 the side of the shell, so that they cannot be seen in 

 tbe position in which this animal is drawn in this- 

 plate. To see the eyes, we should look into the 

 face of the animal, Between the eyes and palpi 

 are the jaws, consisting of a very powerful appa- 

 ratus of moveable appendages. 



The position of the main organs is the more im- 

 portant, as it is reflected by the external covering j 

 so much so, that froro the outside,, ia various 



