184 



CRUSTACEA. 



The Crustacea, regarded under the different rela- 

 tions exhibited by their organisation, ought unques- 

 tionably to occupy an elevated range in the annulose 

 sub-kingdom. They ought not, indeed, to be far re- 

 moved from the Arachnida and insi-cts, which, like 

 themselves, have a symmetrical body encased in a- 

 hard corneous covering, performing the offices of an 

 external skeleton, and with articulated legs, eyes al- 

 ways apparent, sexes distinct, &c. They are, how- 

 ever, evidently much farther removed from the Anne- 

 lida, in which the body is destitute pf true articulated 

 limbs, the eyes wanting, and the generation often her- 

 maphrodite. These last, indeed, in many respects in- 

 ferior both to the Crustacea and Imecta, seem to be 

 more nearly allied to the intestinal worms, and Epizo- 

 aria. With respect again to the fishes with which 

 they are vulgarly associated, their relations are very 

 trifling indeed*; but with the mollusca, especially such 

 as the Cephalopoda, there is much greater affinity, so 

 that they might naturally be placed after them in the 

 series of animals ; but as we find other m.ollusca, such 

 as the Gasteropoda and Acepliala, still less perfectly 

 organised, we should either be compelled to intro- 

 duce the Crustacea, in the midst of them, or to place 

 the mollusca either after the entire group of annulosed 

 animals, as the ancients did, or before the Crustacea, as 

 the more recent zoologists have done ; and this latter 

 step is confirmed by the observations of Latreillc, who 

 has proved the existence of the great affinity between 

 certain of the least perfectly organised fishes, and the 

 most perfectly organised mollusca, as \\\Q Cephalopoda. 



The following are the characters by which the ani- 

 mals of this class are distinguished from the other an- 

 nulosa. Animals destitute of internal vertebra? having 

 white blood ; the body divided into various segments 

 (differing in number), encased in a crustaceous en- 

 velope, and provided with articulated legs, respiring 

 by means of branchiae or branchial plates, ordinarily 

 annexed to the base of the legs or lower jaws, having 

 a distinct heart, furnished with visible vessels, with 

 legs, of which the number is generally five or seven 

 pairs, and always destitute of wings ; head generally 

 confounded with the thorax, furnished with two pairs 

 of antennae, and a pair of mandibles (often palpi- 

 gerous) ; two pairs of lower jaws, and three pairs of 

 foot-jaws (the two posterior pairs of which are trans- 

 formed into two additional pairs of legs, when there 

 are seven pairs of the latter organs) ; mouth also fur- 

 nished with an upper and lower lip, or rather a tongue; 

 the external pair of foot-jaws (see figure of the under- 

 side of the body of Carcinus moenas, vol. i. p. 722,) 

 performing the office of a lower lip ; eyes two, facet- 

 ted, often borne upon footstalks ; sexes distinct ; sexual 

 organs placed either at the base of the legs or the 

 extremity of the body. 



The class which we are now considering may be 

 considered as taking the place in the ocean which in- 

 sects occupy on land. They are very varied in their 

 forms, as well as size, some of them being in fact the 

 giants of the sub-kingdom to which they belong, whilst 

 others are of a microscopic minuteness. Some species 

 are of a globular or oval shape, others square, whilst 



* A celebrated zoologist recently requested a friend who was 

 going a long sea-voyage, to bring him home all the fishes he 

 could procure. "Yes," was the reply," I will catch you all kinds, 

 from a whale to a shrimp;" neither of which, as every person 

 acquainted with the slightest outlines of zoology well knows, 

 are fishes. We mention this anecdote to show that a little know- 

 ledge here would not have been a dangerous thing. 



a few are linear and elongated , some again have the 

 body quite flat, others are compressed, and in a tew 

 the covering of the body, instead of being hard and 

 crustaceous, exists in u softened and menibranaceous 

 state. 



These animals possess the ordinary senses which 

 the inferior animals are gifted with, although, from t'ne 

 great modification in their structure, as compared with 

 that of the vertebrated higher animals, it is difficult, 

 and, indeed, often impossible for us to assign them to 

 their legitimate organs. That they taste is evident 

 from the fact that they are not indifferent to the kind 

 of food which they meet with, and which, indeed, they 

 seek with much assiduity. The very complex organi- 

 sation of their mouths, the development of which far 

 surpasses that of, insects, must doubtless be regarded 

 as affording the seat of this sense. The organs of 

 sight are very distinct, in the greater number existing 

 as facetted eyes, borne upon footstalks, often of a very 

 great length, as in Podopli thalmus; often, however, they 

 are sessile, that is, not elevated upon the surface of 

 the head or shell. The structure of the eyes of these 

 animals has lately been studied very minutely by M. 

 Muller (Ann. Sc. Nat., July 1829). The sense of 

 touch, from the hard envelope in which these animals 

 are encased, must in all probability be greatly dimi- 

 nished, especially in the more crustaceous species. 

 The circumstance that they are provided with two 

 pairs of antennae, as well as several pairs of palpi at- 

 tached to the jaws and foot jaws, seems to favour the 

 opinion that this sense is transmitted by means of 

 these organs. The sense of hearing has not been dis- 

 tinctly proved to be possessed by these animals ; it 

 has, however, been supposed that the seat of this sense 

 was placed in an excavation observed at the base of 

 the external pair of antenna; in certain of the larger 

 species, as the lobster, &c.. in which cavity it has been 

 stated, that a small sac filled with fluid exists, in the 

 midst of which a nervous thread was fixed ; it is cer- 

 tain, however, that the aperture of this cavity is closed 

 by a membrane, and that in some cases th'ere exists 

 (as in Maia} a small crustaceous organ or moveable 

 operculum, the use of which appears to be to distend 

 the membrane which shuts the orifice. Of the distinct 

 existence of the sense of smell we are also equally igno- 

 rant, although from analogy we are led to believe that 

 it is possessed by these animals. Its seat is also un- 

 known, although Rosenthals and Robineau considered 

 that it existed in a pair of small orifices at the base of 

 the internal pair of antennae. 



Annulose animals having the vertebrae as it were 

 external, are necessarily prevented from undergoing 

 a similar mode of growth to that which distinguishes 

 those animals which, having the vertebrae internal, 

 acquire an increase of the external softer portion of 

 the bodv, without any obstacle to its addition ; but. 

 the solid shell of the crab may readily be conceived 

 to be an unsurmountable obstacle to its growth, which 

 nevertheless takes place from year to year. Nature, 

 therefore, has removed this obstacle in a curious 

 manner. The caterpillar casts its skin several times 

 before it becomes a chrysalis, the crab in like manner 

 undergoes an annual moulting ; but it may well be 

 conceived that the complete shedding of a crustaceous 

 envelope like that of the crab, must be a far more 

 remarkable operation than the slipping of a thin 

 membranous skin like that of a caterpillar. The prin- 

 ciple is, nevertheless, the same in both cases. When, 

 however, we consider that not only the shell but also 



