12 PEOTOZOA 



(Fig. 87) or flagella (Fig. 35), which are projections of the ectosarc, are 

 present and are permanent organs of locomotion. In a few ciliates true 

 muscle fibres are present. Sensation is exercised by the entire surface of 

 the body and its projections. 



The inner portion of the body is called the entosarc (Fig. 2) ; in it 

 nutrition is carried on. Food in solid form may be taken into the mass of 

 the entosarc, where it is usually surrounded by a watery fluid forming the 

 food-vacuole, and digested and absorbed. In the lower Protozoa the food 

 particles are taken in through the outer surface by a simple process of 

 engulfing, no mouth being present, but in most Infusoria a definite mouth 

 is present in the ectosare from which a gullet leads into the entosarc. 

 Indigestible portions of the food are thrown out through an anal opening 

 in the ectosarc which in the higher forms alone is a permanent structure. 

 Many protozoans lead parasitic lives and absorb the vital fluids of their 

 hosts through the outer surface of the body. Great numbers of the flagel- 

 lates also closely resemble plants in their habits of nutrition. Respiration 

 is carried on through the entire outer surface of the body, as is also 

 excretion in most marine and parasitic Protozoa. In the majority of 

 Protozoa, however, a special excretory organ, the contractile or pulsating 

 vacuole (Fig. 2, cv), is present in the form of a minute globule of clear, 

 excretory fluid which collects periodically and is then discharged to the 

 outside through a temporary opening in the ectosarc. The contractile 

 vacuole probably exercises a respiratory as well as an excretory function, 

 carbon-dioxide being eliminated by its discharges. 



The characteristic method of reproduction is by equal division. The 

 nucleus takes the lead in the process and is quickly followed by the body 

 of the cell, and two new individuals are thus formed from a single old 

 one. In many Protozoa the new individuals are not completely separated 

 from each other, but remain connected together, and a colony is thus 

 formed, while in some a physiological division of labor occurs among the 

 members of such a colony and an important step towards the development 

 of a metazoan animal is taken. Still another modification of simple 

 division is the formation of spores, which characterize the Sporozoa and 

 occur occasionally in the other Protozoa. Spore formation may be pre- 

 ceded by the encystment of the animal and a period of rest; the animal 

 draws itself together into as small a compass as possible and then secretes 

 a firm membrane or shell within which it lies while spore formation is 

 being accomplished. Later the cyst breaks and the spores being liberated 

 each becomes a new individual. 



Of universal occurrence among Protozoa is conjugation, or the tem- 

 porary or permanent fusion of individuals, which in some of its phases 

 resembles the process of fertilization in the higher animals. The two 



