ABRANCHIATA. 303 



They live in fresh water, in the mud, and in inundated grounds which 

 they perforate in every direction. 



The different species are not yet well distinguished; the most common, 

 Gordius aquaticus, L., is several inches in length, almost as fine as a hair, 

 and brown, with blackish extremities. 



CRUSTACEA, ARACHNIDES, AND INSECTA: 



OR ARTICULATED ANIMALS WITH ARTICULATED FEET. 



These last three classes of the Articulata, which were united by 

 Linnaeus under the general name of Insecta, are distinguished by at 

 least six articulated feet. Each articulation is tubular, and contains 

 the muscles of the succeeding one, which always moves by gyngly- 

 mus, that is, in but one direction. 



The first articulation, which attaches the foot to the body, and 

 which is composed of two pieces, is called the coxa', or hip; the 

 following one which is, usually, nearly in a horizontal position, the 

 femur, or thigh; and the third, generally vertical, the tibia or leg. 

 To these ensues a suite of small ones which touch the ground, form- 

 ing the true foot, or what is denominated the tarsus. 



The hardness of the calcareous or horny envelope of the greater 

 number of these animals, is owing to that of the excretion which is 

 interposed between the dermis arid epidermis, or what is termed in 

 man the mucous tissue. This excretion also contains the bril- 

 liant and varied colours with which they are so often decorated. 



They are always furnished with eyes, which are of two kinds: 

 simple eyes or ocelli, which resemble a very minute lens, generally 

 three in number, and arranged in a triangle on the summit of the 

 head; and compound eyes, where the surface is divided into an in- 

 finitude of different lenses called facets, to each of which there is a 

 corresponding filament of the optic nerve. These two kinds may 

 be either united or separated according to the genus. 



Other organs which for the first time are here presented to us, 

 and which are found in two of these classes (the Crustacea and the 

 Insecta), the antennas, are articulated filaments, varying greatly in 



