32 INVERTEBRATE ANIMALS. 



The Amoeba possesses no breathing-organs, of any kind, 

 and no excretory organs, so that these functions must be per- 

 formed by the general surface of the body in a manner some- 

 what the same as the exhalation from the skin which takes 

 place in the higher animals. There are also no traces of a ner- 

 vous system, and no organs of sense, and the only other struct- 

 ure of any kind is what is known as the nucleus. The nucleus 

 is simply a small rounded or oval granular mass, and there may 

 be more than one in the same individual. Its function, how- 

 ever, is quite unknown, though it is probably connected with 

 reproduction. The means employed by the Amoeba to per- 

 petuate the species are various, but the only one which need 

 be mentioned is the process by self-division. This is what is 

 technically called " fission " (Lat findo, I cleave), and it con- 

 sists in a gradual division or cleavage of the body into two 

 parts, each of which then becomes a separate and independent 

 individual. In some cases this process is slightly varied, a 

 single pseudopodium alone being cast off and becoming a 

 fresh Amoeba, but this does not differ essentially from the for- 

 mer. 



Regarding the Amceba from a physiological point of view, 

 we see that, though the animal nourishes itself and maintains 

 its existence perfectly, the process of nutrition is carried on in 

 the simplest possible manner and with the simplest possible 

 apparatus. There is no permanent mouth, no stomach or ali- 

 mentary canal of any kind, no respiratory or excretory organs, 

 and even no distinct aperture for the extrusion of indigestible 

 food. The only distinct structure which is at all concerned in 

 nutrition is a rudimentary contractile cavity, the first foreshad- 

 owing of the heart in the higher animals. As regards the 

 functions of relation, it is questionable how far the Amoeba 

 can be said to have distinct perceptions or sensations of any 

 kind. It has no nervous system or organs of sight or hearing, 

 and in all probability it has nothing more than a general sen- 

 sibility to light. It appears, however, to be fully aware when 

 any object comes in contact with a pseudopodium, and even 

 to have some idea whether this is fit for food. Locomotion, 

 as we have seen, is entirely effected by the temporary pro- 

 cesses of sarcode or pseudopodia, and there are no permanent 

 organs set aside either for locomotion or for prehension that 

 is, for seizing external objects. 



The only other member of the Amoebea which deserves 

 notice is the Difflugia (Fig. 3, 5), which is not uncommonly 

 found in fresh water. Difflugia in its essential structure 



