INTRODUCTION. 35 



Animalia Mollusca, 



Or soft animals. Although, as respects the external configura- 

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

 uniform as that of the Vertebrata; 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 longitudi- 

 nally through the abdomen, dilated at intervals into knots or gan- 

 glions. The first of these knots, placed over the oesophagus, and 

 called brain, is scarcely any larger than those that are along the ab- 

 domen, 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 

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

 We will call these animals 



Animalia Articulata, 



Or articulated animals, in which is observed the transition from 

 the circulation in closed vessels, to nutrition by imbibition, and the 

 corresponding one of respiration in circumscribed organs, to that 

 effected by trachea or air-vessels distributed throughout the body. 

 In them, the organs of taste and sight are the most distinct; one 

 single family alone presenting that of hearing. Their jaws, when 

 they have any, are always lateral. 



The fourth form, which embraces all those animals known by the 

 name of Zoophytes, may also properly be denominated 



Animalia Radiata, 



Or radiated animals. We have seen that the organs of sense and 

 motion in all the preceding ones are symmetrically arranged on the 

 two sides of an axis. There is a posterior and anterior dissimilar 

 face. In this last division, they are disposed like rays round a cen- 



