170 UNICELLULAR ANIMALS. 



about by currents in the entoplasm), and two very large contrac- 

 tile vacuoles (c.v) occupying a constant position, one near either 

 end of the body. The nucleus (as in Infusoria generally) is 

 differentiated into two distinct parts, viz. , a large oval macro- 

 nucleus (mac.) and a much smaller spherical micronucleus (mic.) 

 (double in some species) lying close beside it. 



Unlike Amoeba, ParamoBcium possesses a distinct mouth (m) 

 and o&sophagus (ce) which open to the exterior through an oblique 

 funnel-shaped depression known as the vestibule (y) situated at 

 one side of the body. Minute floating food-particles are drawn 

 by the cilia into the mouth and accumulate in a ciliary vortex at 

 the bottom of the oesophagus. From time to time a bolus or 

 food -mass is thence passed bodily into the substance of the en- 

 toplasm, forming a food-vacuole within which digestion takes 

 place. The indigestible remnants are finally passed out not 

 through a permanent opening or anus, but by breaking through 

 the protoplasm at a definite point, hence known as the anal 

 spot, which is situated near the hinder end (Fig. 87). The 

 contractile vacuoles of Pa/ramo&cium are especially favorable for 

 study, showing at the moment of contraction, or just before it, 

 a pronounced star-shape, with long canals running out into the 

 protoplasm. Through these liquid is supposed to flow into the 

 vacuole. 



Like Amoeba, Paramo3cium occurs both in an active and in 

 an encysted state. In the former state it multiplies by trans- 

 verse fission, division of both macronucleus and micronucleus 

 preceding or accompanying that of the protoplasmic body (Fig. 

 88, A). Under favorable conditions division may take place once 

 in twenty-four hours, or even oftener. This process, which is a 

 typical case of agamogenesis, may be repeated again and again 

 throughout a long period. But it appears from the celebrated 

 researches of Maupas that even under the most favorable con- 

 ditions of food and temperature the process has a limit (in the 

 case of Stylonichia, a form related to Paramwcium, this limit 

 is reached after about 300 successive fissions). As this limit is 

 approached the animals become dwarfed, show various signs of 

 degeneracy, and finally become incapable of taking food. The 

 race grows old and dies. 



In nature, however, this limit is probably seldom if ever 



