INTRODUCTION. 31 



merely attached to the skin, which constitutes a soft contrac- 

 tile envelope, in which, in many species, are formed stony 

 plates, called shells, whose position and production are ana- 

 logous to those of the mucous body. The nervous system is 

 contained within this general envelope along with the viscera, 

 and is composed of several scattered masses connected by ner- 

 vous filaments; the chief of these masses is placed on the 

 oesophagus, and is called the brain. Of the four senses, the 

 organs of two only are observable, those of taste and sight, the 

 latter of which are even frequently wanting. One single 

 family alone presents organs of hearing. There is always, 

 however, a complete system of circulation, and particular or- 

 gans for respiration. Those of digestion and secretion are 

 nearly as complex as in the vertebrata. We will distinguish 

 the animals of this second form by the appellation of 



Animalia Mollusca. 



Although, as respects the external configuration of the 

 parts, the general plan of their organization is not as uniform 

 as that of the vertebrata j there is always an equal degree of 

 resemblance between them in the structure and the functions. 



The third form is that remarked in worms, insects, &c. 

 Their nervous system consists of two long cords, running lon- 

 gitudinally through the abdomen, dilated at intervals into 

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

 oesophagus, and called brain, is scarcely any larger than those 

 that are along the abdomen, with which they communicate by 

 filaments that encircle the oesophagus like a necklace. The 

 covering or envelope of the body is divided by transverse 

 folds into a certain number of rings, whose teguments are 

 sometimes soft, and sometimes hard; the muscles, however, 

 being always situated internally. Articulated limbs are fre- 

 quently attached to the trunk ; but very often there are none. 

 We will call t^hese animals 



Animalia Articulately 

 Or articulated animals, in which is observed the transition 



