INTRODUCTION. 33 



which constitutes a soft, contractile envelope, m which, in many species, are formed 

 stony plates, called shells, the production and position of which are analogous to that 

 of the mucous body ; the nervous system is contained within this general envelope, 

 together with the viscera, and is composed of several scattered masses, connected by 

 nervous filaments, and of which the principal, placed over the oesophagus, bears the 

 name of brain. Of the four senses, the organs of those of taste and vision only can be 

 distinguished ; the latter of which are even frequently wanting. A single family 

 alone presents organs of hearing. There is always, however, a complete system of 

 circulation, and particular organs for respiration. Those of digestion and of the secre- 

 tions are little less complicated than in the vertebrated animals. We will distinguish 

 the animals of this second form by the appellation of 



Molluscous Animals {Anhnalia mollusca). 



Although the general plan of their organization is not so uniform, as regards the 

 external configuration of the parts, as that of the vertebrates, there is always an equal 

 degree of resemblance between them in the essential structure and the functions. 



The third form is that observed in insects, worms, &c. Their nervous system con- 

 sists of two long chords running longitudinally through the abdomen, dilated at inter- 

 vals into knots or ganglions. The first of these knots, placed over the oesophagus, 

 and called brain, is scarcely any larger than those which are along the abdomen, with 

 which it communicates by filaments that encircle the oesophagus like a collar. The 

 envelope of their trunk is divided by transverse folds into a certain number of rings, of 

 which the teguments are sometimes hard, sometimes soft, but to the interior of which 

 the muscles are always attached. The trunk often bears on its sides articulated 

 limbs, but is frequently unfurnished with them. We will bestow on these animals 

 the term 



Articulate Animals {Animalia urticulata). 



It is among these that the passage is observed from the circulation in closed vessels, 

 to nutrition by imbibition, and the corresponding transition from respiration in cir- 

 cumscribed organs, to that effected by trachcce or air-vessels distributed through tlie 

 body. The organs of taste and vision are the most distinct in them, a single family 

 alone presenting that of hearing. Their jaws, when they have any, are always lateral. 



Lastly, the fourth form, which embraces all those animals known under the name of 

 Zoophytes, may be designated 



Radiate Animals {Animalia radiata). 



In all the preceding, the organs of sense and motion are arranged symmetrically on 

 the two sides of an axis. There is a posterior and an anterior dissimilar face. In this 

 last division, they are disposed as rays round a centre ; and this is the case, even when 

 they consist of but two series, for then the two faces are alike.* They approximate to 

 the homogeneity of plants, having no very distinct nervous system, nor organs of 

 particular senses : there can scarcely be perceived, in some of them, the vestiges of a 



* M. A^jassir, tins expressed a different opinion. Sec Radinta.— V.n. 



