ARTICULATA. 293 



by infinite ramifications to every part of the body. A vestige only 

 of a heart is perceptible, consisting of a dorsal vessel which experi- 

 ences an alternate contraction and dilatation, but to which no branch 

 has ever been discovered, so that we are forced to believe that nu- 

 trition is effected in this class of animals by imbibition. It is, pro- 

 bably, this sort of nutrition which necessitated the kind of respiration 

 proper to Insects; for as the nutritive fluid is not contained in vess- 

 els, and could not be directed towards pulmonary organs in search 

 of air, it was requisite that this air should be diffused throughout 

 the body to reach the fluid. This is also the reason why Insects 

 have no secretory glands, but are provided with mere spongy vess- 

 els, which, by the extent of their surface, appear to absorb the pe- 

 culiar juices they are to produce, from the mass of the nutritive fluid. 



Insects vary infinitely as to the form of the organs of the mouth, 

 and those of digestion, as well as in their industry and mode of life. 



The Crustacea and Arachnides were long united with the Insecta 

 under one common name, and resemble them in many points of 

 their external form, in the disposition of their organs of motion, and 

 of the sensations, and even in those of manducation. 



CLASS I. 

 ANNULATA. 



The Annulata are the only invertebrate animals that have red 

 blood. It circulates in a double system of complicated vessels. 



Their nervous system consists in a double knotted cord, like that 

 of Insects. 



Their body is soft, more or less elongated, and divided frequently 

 into a considerable number of segments, or at least of transverse 

 plicae. 



They nearly all inhabit the water the Lumbrici or Earth-worms 

 excepted; several penetrate into holes at the bottom, or construct 

 tubes there with the ooze or other matters, or even exude a calca- 

 reous substance, which envelopes them with a sort of tubular shell. 



