86 ) 



CHAPTER VI. 



PREPARATION OF FOOD. 



1. Prehension of Liquid Food. 



IN studying the series of processes which constitute assi- 

 milation, our attention is first to be directed to the mode in 

 which the food is introduced into the body, and to the me- 

 chanical changes it is made to undergo before it is subjected 

 to the chemical action of the digestive organs. The nature 

 of these preliminary processes will, of course, vary accord- 

 ing to the texture and mechanical condition of the food. 

 Where it is already in a fluid state, mastication is unneces- 

 sary, and the receiving organs consist simply of an appara- 

 tus for suction. This is the case very generally with the 

 Entozoa, which subsist upon the juices of other animals, 

 and which are all provided with one or more sucking ori- 

 fices, often extended in the form of a tube or proboscis.* 

 The Hydatid, for instance, has four sucking apertures dis- 

 posed round the head of the animal: the Taenia has orifices 

 of this kind in each of its jointed segments: the Jlscaris and 

 the Earth-ivorm have each a simple mouth. The margin 

 of the mouth is often divided, so as to compose lips; of these 

 there are generally two, and in the leech there are three. 

 In some rare cases, as in the Planaria, there is, besides the 



* Some species of Fasdohs, or flukes, are furnished with two, three, six, 

 or more sucking disks, by which they adhere to surfaces: to these animals 

 the names Distoma, Tristoma, Hexastoma, and Polystoma have been given; 

 but these denominations, implying 1 a plurality of mouths, are evidently in- 

 correct, since the sucking- disks are not perforated, and do not perform the 

 office of mouths; and the true mouth for the reception of food is single. 

 Cuvier discovered an animal of this class furnished with above a hundred of 

 these cup-shaped sucking organs. See Edinburgh Philos. Journal, xx. 101. 



