408 



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 branchiae, and thence back to the heart. These branchiae 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 (Cymothoa, Phyllosoma, 

 Palinurus, Palomon, 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 upper 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 primitive 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 are 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 distribution, 

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

 observations hitherto made upon the nervous system. 



* [The modifications in the structure of the nervous system of the 

 larva, pupa, and imago of the same insect, fully confirm this, that of 

 the larva resembling that of the Talitrus, whilst that of the imago is 

 more analogous to that of the Crab. If we regard the larva as in a 

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

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

 is the station generally assigned to it, without reference to its nervous 

 system.] 



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

 Zoological Researches and other more recent articles, this writer 

 asserting that the Crustacea undergo a series of transformations as 

 striking as those of the true insects; the anomalous animals long 

 known under the generic name of Zoea, and which have long perplexed 



Crustaceologists {for want of a perfect investigation of their struc- 

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

 Decapoda. In some cases, however, where- a minute analysis of the 

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

 obtained, Rathke having dissected the eggs and watched the gradual 

 developeraeut of the embryo of the crayfish, and I 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.] 



t Whence Lamarck divided the Crustacea into the Pediocles (or eyes 

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

 names (applying them only to the Malacostraea) into Podopthalma and 

 Edriopthalma. Grouovius first employed this eharacte*. 



