408 



Div. 3. ARTICULATED ANIMALS. 



double, and analogous to that of the Mollusca. The blood is transmitted from the 

 heart, situated near the back, to the different parts of the body, where it is conveyed 

 to the branchia;, and thence back to the heart. These branchiee are situated either at 

 the base of the legs or upon the legs themselves, or upon the subabdominal ap- 

 pendages, forming either pyramidal masses, composed of layers of fine plates or clothed 

 with setae, or consisting of simple plates in tufts, even in some appearing to consist 

 only of hairs. 



The nervous system of the Crustacea (especially investigated by Cuvier, Audouin, 

 and Milne Edwards), exhibits two very different appearances, constituting the two 

 extremes of the modifications it presents in this class. Sometimes, as in the leaping 

 shrimps {Talitrus), it is composed of two nervous chords, with knots or ganglions 

 at equal distances along the whole length of the body, and sometimes, as in the Crab 

 (Maia Squinado), it consists of only two nervous masses, of unequal size, one placed in 

 the head and the other in the thorax. Other Crustacea (Ct/mothoa, Phyllosoma, 

 Paliniirus, Palemon, and Astacus), exhibit intermediate formations, showing the 

 gradual modifications.* 



The Crustacea are destitute of wings, provided with two facetted eyes, but rarely 

 with simple eyes, and generally with four antennae. They have in general (the 

 Poecilopoda excepted) three pairs of maxillae (the upj)er pair or true mandibles included), 

 the same number of foot-jaws, the outer pairs of which become, in many species, real 

 feet ; and ten legs, all of which are terminated by a single hook. When the two 

 outer pairs of foot-jaws perform the office of feet, the number of legs is [increased to] 

 fourteen. The mouth consists, as in insects, of an upper lip, a tongue, but no true 

 lower lip comparable with that of insects, the external pair of foot-jaws [the third 

 pair, or, where the two outer pairs become legs, the first pair] closing the mouth and 

 acting instead of a lip, [thus proving what has been suggested above relative to the 

 nature of the labium in insects]. 



Their envelope is generally solid, and more or less calcareous. They change their 

 coats several times, generally retaining their jjriniitive formf and their natural activity. 

 They are in general carnivorous, aquatic, and their life extends through several years. 

 They do not become adults until after a series of moultings. With the exception of a 

 small number in which these moultings somewhat modify the primitive form, and 

 augment the number of locomotive organs, these animals arc at their birth (size 

 excepted) such as they will remain throughout their life. 



The situation and the form of the branchiae, the manner in which the head is 

 articulated with the trunk or thorax, the moveable or fixed structure of the eyes^, the 

 organs of mastication, and the tegumentary system, form the bases of our distril)ution, 

 and give rise to the following orders in the class, and which are confirmed by the 

 observations hitherto made upon the nervous system. 



• [The modification* in the Htructure of tlie nervous Rystcin of the 

 larvK, pupa, and iinn)(i) of t)ie Knme insect, fully confirm thin, that of 

 the larva resembling that ol the I'lilitrus, whilst that of the hnago ti> 

 more analogous to that of the Crab. If we rcxard the laria as in a 

 state of immaturity or imperfection, we should be led to consider the 

 Crab as far higher in the chain of nature than the Talitrus, and such 

 is the station j^cnerally assit^ned to it, without reference to its nervons 

 gystcnu] 



t [This statement has been opposed by Dr. J. V Thompson, in his 

 Z4>olo^cal Rcseiirches and other more recent a.-ticlcs. this writer 

 assertintf that the Crustacea under{(0 a series of transformations as 

 striiting as those of the true insects ; the anotnalous animals long 

 llDOwn under thegcneric name of Zoea, and which have lung perplexed 



Crustaceolokjists (for want of a perfect investigation of their struc- 

 ture), being alhrmeil by him to be the young of the Crabs and other 

 Decapoda. In some cases, however, whcrC' a minute analysis of the 

 eggs of different species has been made, a contrary result has been 

 ojjtained, Rathke liavlng dissected tlie eggs and watched the gradual 

 developcnient of the embryo of the crayfish, and 1 having dissected 

 the eggs of the land crab of the West Indies, the young in both in* 

 stances (and in others subsequently observed by Rathke) resembling 

 the parents in general appearance.] 



J Whence Lainarcit divided the Crustacea into the Pediocles (or eyes 

 on footstalks) and Sessiliocles (or sessile eyes). Leach changed these 

 names (applying tliem only to the Malacostraca) into Pudopthalina and 

 Edriopthalma. Groiiovius 6rst employed this character. 



