' [Tlie morliSciilii-ii!. in tl.c structure of llm nervous fiy.stem of tlie 

 Irir'.-ii, impa, and imiigii of tlie same insttt, fully confirm thia, thai of 

 tlic Irirva rcueinbllnfir that of tlii; Talitrus, whilst that of the imatjo is 

 mure iiiiuloj,^ous to that of the Crab. If we regard the lar'-a us in a 

 state of iraiiiaturitj' or imperfection, we should be led to consiiler the 

 Crab rxH far higher in the L-hain of nature than the Talitrus, and siieh 

 is the station geierally assitrned to it, without reference to its nervous 

 ayslem.] 



t [Tliis statement has been ojiposed by Ur. J. V. Thompson, in liii 

 2'J0io({ical 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 annmrvlnus animaln lon|r 



Crustaceologists (for want of a perfect investijjation of their struc- 

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

 Decapoda, In some cases, however, wherC' a minute analysis of tlie 

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

 obtaiucil, Rathke having dissected the e^ga and watched the gradual 

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

 the eggs of the land crab of the West liidies, the young in boili in- 

 stances (and in others subse(|uently observed by RutUke) reacmldiiig 

 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 

 les (applying them only to tlic Malacostrnca) into Pndopthalma and 



403 ARTICULATED ANIMALS. 



double, and analogous to tliat of the MoUusca. The blood is transmitted from the 

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

 to the bninohiae, and thence back to the heart. These branchiie are situated either at 

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

 pendages, forming cither pyramidal masses, composed of layers of fine pjlates 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), exliibits two very difterent appearances, constituting the two 

 extremes of the moditicatiuns it jircsents in this class. Sometimes, as in the leaping ] 

 shrimps {Talilriis), it is compiosed of two nervous chords, with knots or ganglions j 

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

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

 the head and the other in the thorax. Other Crustacea {Cymotlioa, Flujtlosoma, 

 Paluiiinis, Paleiiion, and Astacus), exhibit intermediate formations, showing the 

 gradual modifications.* 



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

 with simple 03-05, and generally with four antenna?. They have in general (the 

 Pcccilopoda excepted) three pairs of maxilla; (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]. 



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

 coats several times, generally retaining their primitive formf and their natural activitjr. 

 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 nf the branchia", the manner in which the head is 

 articulated with the trunk or thorax, the moveable or fixed structure of the eyesj:, the 

 organs of mastication, and the tegurnentary sj-stem, form the bases of our distribution, 

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

 observations hitherto made up.on the nervous system. 



iidcrtlie (,'r.iH.'riu iiaruc uC Zui^a, ami ^^■llLL■li lii.v^- luiif; perplc-\LMj j EilrmiUhnlmii. Gruiinviiis lirst uiiiploy-.- 



