26 INVERTEBRATE ANIMALS. 



substance {called " sarcode"), showing no composition out of 

 distinct segments, having no distinct internal cavity, no ner- 

 vous system, and either no organs devoted to digestion, or at 

 best a very rudimentary alimentary apparatus. 



Of all tlie points enumerated in this definition as charac- 

 teristic of the Protozoa, none is more important than the na- 

 ture of the body-substance. The body in all known Protozoa 

 is composed of a substance which is generally known by the 

 name of " protoplasm " — or, better, " sarcode " (Gr. sarx, flesh ; 

 eidos, form). This sarcode is a gelatinous substance, very like 

 white-of-egg to look at, and really of nearly the same chemical 

 constitution, consisting mainly of albumen, or of some body 

 allied to albumen. Generally, however, it contains nimierous 

 oil-globules scattered through it. The sarcode shows the phys- 

 iological property of " contractility " — that is to say, under 

 appropriate stimuli, or at the will of the animal, it may be 

 made to contract or shorten its dimensions, thus giving rise 

 to movements. As a rule, no other structures appear in the 

 sarcode except minute rounded particles, or granules and 

 molecules, but in some cases larger definite structures are 

 formed out of it. Of this nature is the so-called " nucleus " 

 found in many Protozoa. 



As regards their internal structure, some Protozoa exhibit 

 nothing worthy of the name of structure at all, the entire 

 body being simply composed of sarcode, containing scattered 

 granules (for example, the Foraminifera). In other cases 

 there are found certain definite bodies which are known as the 

 " nucleus " and " nucleolus," and which are usually, if not 

 always, connected with reproduction. Very often, too, there 

 are found certain minute cavities or chambers which close and 

 expand at definite intervals, and which are known as the 

 " contractile vesicles." These are, doubtless, rudimentary or- 

 gans of circulation. In one division of the Protozoa (the In- 

 fusoria) there is a permanent mouth and a short gullet, but 

 in all the others there are no definite organs connected with 

 the process of digestion. In no Protozoan, however, without 

 exception, have any traces of a nervous system been hitherto 

 detected ; and in none, even in those which possess a mouth, 

 is there any distinct and definite cavity or chamber within the 

 body in which the particles of food are received. No organs 

 of sense exist in any of the Protozoa — that is to say, there 

 are no distinct organs fitted for the reception of impressions 

 produced by light or sound ; but the general surface of the 

 body appears capable of receiving the impressions produced 



