OF ANIMALS. 35 



gous to the external one. The animals which present the 

 first rudiments of the arrangement, are also the polypi and 

 acalepha and some intestinal worms. In other animals of the 

 same class, the gastric cavity has prolongations extending into 

 the mass of the body, in order to provide it with nourishment. 

 In some acalepha and intestinal worms, the stomach is want- 

 ing, and there are only ramified prolongations opening on the 

 external surface. In all these first appearances of an intestinal 

 cavity, that cavity is confined to a mere elongated sac, having 

 one single opening. Several of the echinodermata, and in- 

 testinal worms have a distinct intestinal canal, a mouth and an 

 anus, an arrangement which is to he observed in all the higher 

 classes, in which this canal, more or less enlarged, or more or 

 less contracted, is extended through the body. The existence 

 of this canal is perceived at the same time with the cylindri- 

 cal and elongated form of the body. 



The mouth presents several varieties, the principal of which 

 are those of a simple orifice, or an opening furnished with 

 muscles, and sometimes with hard parts, but intended for suc- 

 tion only; or an orifice surrounded with muscles, and furnish- 

 ed with hard parts to divide the aliment. 



20. In many of the inferior animals, the nourishing fluid, 

 absorbed by the parietes of the intestines, which are either 

 simple or elongated and extended in the body by ramified ap- 

 pendages, is carried immediately by the areolar substance into 

 every part of it. This is the case with all the radiated ani- 

 mals, and with the immense class of insects. In fact, in no 

 insect are there any vessels, and the nourishing fluid must 

 pass by imbibition from the intestine into every part of the 

 body; there is only a dorsal vessel which appears to be the 

 rudiment of a heart, but there are no branches for circula- 

 tion. 



In animals of a still higher class, the nourishing liquid, ab- 

 sorbed by the parietes of the intestines, circulates in close ves- 

 sels, the minute ramifications of which, only permit the nour- 

 ishing molecules to pass into the substance of the body. The 

 vessels which go from the centre of the circulation to all the 

 other parts are called arteries; those which bring back the li- 



