242 SUPPLEMENT 



the uiollusca, over the annelides,and over all the animals which 

 have been placed subsequently to the articulated classes. 



They have the greatest resemblance to the arachnida, since 

 they possess, in the same degree of energy, the first two 

 animal functions of which we have just spoken. The 

 arachnida also exhibit an additional relation with the Crus- 

 tacea, namely, that which results from the presence of a 

 heart, or centre of circulation, communicating with the assem- 

 blage of vessels, destined to carry the nutritive fluid or lymph, 

 into the various parts of the body. 



The organs of locomotion in the Crustacea, consist, 1st. of 

 passive organs performing the functions of the skeleton, in 

 vertebrated animals, and principally composed of the external 

 skin, which is hardened and divided into segments, or por- 

 tions of segments more or less complicated, for the body and 

 limbs, but always symmetrical ; 2d. of active organs, soft and 

 fibrous, or muscles which are contractile by the agency of the 

 nervous system. 



The solid pieces are articulated together, either with, or 

 without motion. Those which are in the first predicament, 

 such as the plates composing the breast-piece of crabs and 

 astaci, are distinct only by straight sutures. Those which 

 are in the second, ordinarily move, one upon the other, by 

 ginglymus, or a hinge-like articulation. The mobile parts of 

 the Crustacea are those which we have already described, or 

 which are described in the text, such as antennae, the parts of 

 th« mouth, the peduncles of the eyes, &c. 



The muscles in the Crustacea, as in the insects, are formed 

 of fibres not adherent to each other, not united by a cellular 

 tissue, and not enveloped by aponeuroses, or tendinous ex- 

 pansions. These muscles are numerous, and always placed 

 under or within the solid parts, and disposed so that each ar- 

 ticulation in ginglymus, has its flexor, and its extensor muscle. 



