30 ELEMENTS OF BIOLOGY. 



form of heart ; but it is not quite clear that the function of 

 this little hollow sphere is as above stated. Organs by 

 which the injurious products of the death of the tissues may 

 be eliminated, are absolutely wanting ; and respiration, if it 

 can be said to exist at all as a distinct function, is simply 

 effected by the general surface of the body, and not by any 

 distinct breathing organ. It follows from this that not only 

 is the entire process of nutrition in the Amceba of the very 

 simplest character, but that the process is carried out with 

 the utmost possible absence of complication, and with the 

 very simplest machinery. 



In a Ccelenterate animal, such as one of the sea-anem- 

 ones (fig. 5), the function of nutrition has not increased 

 much in complexity, but the means for its performance are 

 somewhat more specialised. A distinct and permanent 



Fig. 5. A, Actinia mesembryanthemum, one of the Sea-anemones (after Johnston); 

 B, Section of the same, showing the mouth (a), the stomach (l>), and the body- 

 cavity (c). 



mouth is now present, and this is surrounded by a number of 

 prehensile processes or " tentacles," which are of a perma- 

 nent nature, and are not produced for the occasion as in the 

 case of the temporary arms of the Amoeba. The mouth 



