PROTOZOA. 27 
segments, having no distinct internal cavity, no nervous system, 
and etther no organs devoted to digestion, or at best a very 
rudimentary alimentary apparatus. 
Of all the points enumerated in this definition as characteris- 
tic of the Profozoa, none is more important than the nature of 
the body-substance. The body in all known Protozoa is com- 
posed of a substance which is generally known by the name of 
“protoplasm ”—or, better, ‘‘sarcode” (Gr. sara, flesh ; ecdos, form). 
This sarcode is a gelatinous substance, very like white-of-egg to 
look at, and really of nearly the same chemical constitution, con- 
sisting mainly of albumen, or of some body allied to albumen. 
Generally, however, it contains numerous oil-globules scattered 
through it. The sarcode shows the physiological 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. Asa rule, no other 
structures appear in the sarcode except minute rounded parti- 
cles, or granules and molecules, but in some cases larger defi- 
nite structures are formed out of it. Of this nature is the so- 
called ‘‘ nucleus” found in many Prodozoa. 
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 /ovamznifera). 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 organs of circulation. Inone 
division of the Protozoa (the Jnfusoria) there is a permanent 
mouth and a short gullet, but in all the others there are no defi- 
nite organs connected with the process of digestion. In no 
Protozodn, 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 Pro- 
tozoa—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 im- 
pressions produced by contact with foreign bodies, and there- 
