32 INTRODUCTION. 



restoring health by the well directed employment of external 

 agents, or the science of medicine. 



Such are the most general characteristics of animals; but 

 these beings present in their organs and functions a multitude 

 of varieties or of degrees of complication, that it is important 

 to examine. 



17. The external form, or configuration, which may 

 give an idea of the structure, of which it is, in a manner, 

 the outline, presents the following varieties. Some animals 

 are punctiform or globular, as the monads; others are fili- 

 form as the vibrio; some are flat, resembling a small mem- 

 brane, such are the cyclida; finally, others belonging, like 

 the preceding ones to the class of infusoria, have no deter- 

 mined form, their configuration changing at every moment in 

 the most singular manner, these are the protei. These ele- 

 mentary forms, which pertain to all the animals that are the 

 simplest in their composition, are to be found in some indi- 

 viduals of a nobler order, and in certain parts of all others. 

 The same is the case with the stellated or radiated form which 

 belongs to a certain number of classes of animals, and that 

 we meet with in various parts of those animals which have a 

 very different external configuration. 



The radiated form begins to be observed in the order 

 rotiferce, and other polypi; in the acalepha and echinoder- 

 rnata, the radiated form is not confined to their exterior, 

 which resembles a radiated flower, or to a star, but all the 

 parts are arranged around an axis, and on a greater or small- 

 er number of radii. In some other animals the axis being 

 longer, the radiated form becomes cylindrical. The cylindri- 

 cal echinodermata, intestinal worms and annelides establish 

 this passage from the radiated form, of which they still pre- 

 serve some slight marks, to the symmetrical form and articu- 

 lar arrangement which they likewise possess; and the tunicata 

 the transition from the radiated to the symmetrical form with- 

 out articulation. 



The symmetrical form is to be observed, with some few ex- 

 ceptions, in all other animals. In those which have this con- 

 figuration, the body is divided into two lateral parts, or into 



