PROTOZOA OR SIMPLEST ANIMALS. 
14a 
or tissues. Mention may conveniently be made here, also, of Sleeping 
Sickness and Tsetse disease, caused by the presence in the blood, 
respectively of man and of domestic animals, of a parasite (Try- 
panosoma) belonging to the Flagellata and allied to the Sporozoa. 
General Characters of the Group. 
The Sporozoa, from their parasitic habit of Fig. 10a. 
passively absorbing the juices of their host, lack 
organs for capturing and digesting food. Usually 
they are of more or less definite form, and with 
the cell body bounded by a cuticle. Generally 
they are stationary, though some move by means 
of pseudopods (like Amoebm), and some have 
flagella during a phase of their life cycle. The 
most important character, and the one to which 
the group owes its name, is that of forming 
spores or germs, which are commonly enclosed 
each in a tough envelope or cyst (Fig. 10b), and 
often all the spores formed from one parent cell 
are enclosed in a common cyst. The conditions 
necessary for the existence of these organisms are 
so nicely adjusted that each species of parasite is 
usually confined to the same or an allied species 
of host, and to the same tissues of that host. 
The Sporozoa may be grouped under the fol- 
lowing headings : — (1) Gregarines, (2) Coccidia, 
(3) Myxosporidia and Sarcosporidia, and (4) Para- 
sites of the blood. 
(1) Gregarines. These organisms are of very 
common occurrence in the intestine and body 
cavity of invertebrate animals. 
Porospora gigantea (Fig. 10a), from the 
intestine of the lobster, is an unusally large 
species. Specimens, which may be nearly an inch 
in length, look like small worms, yet in spite of 
such huge dimensions they consist of only a 
single cell with a nucleus. The cell body in 
this and some other species is divided into two 
segments, the upper one being small and often 
modified so as to become an efficient fixing organ. 
Monocystis agilis is very commonly found within the sperm sacs 
of earth worms. The little elongated oval Gregarine lives in its 
Porospora gigantea , 
X 150, 
from intestine of 
Lobster. 
(After van Beneden, 
from Lankester.) i 
