CASLENTERATA. 935 
granules. On one side is a slight depression, or “mouth,” 
leading to a short, funnel-shaped throat. A mouth and a 
rudimentary digestive cavity are the distinctive features 
of these Protozoans. They multiply so rapidly (chiefly 
by self- division), that a Paramecium, the most common 
form, may become the parent of 1,364,000 in 42 days. 
There are two main groups: /VZagellata, or Monads, 
provided with one or two flagella, or long bristle - like 
cilia; and Cilzata, which are furnished with numerous 
vibratile cilia. 
Crass 1V.— Spongida. 
An ordinary Sponge is a compound animal, or, more 
properly, an aggregated colony of individual cells, sup- 
ported on a skeleton of horny fibres, which are so united 
as to form a net-work of tubes. The essential part is the 
glairy, gelatinous substance investing this elastic frame- 
work. It consists of myriads of mouthless, sarcode bodies, 
which in some respects resemble Amcebze, but approach 
Fia. 184.—Hypothetical Section of a Sponge: a, superficial layer; 6, inhalent pores; 
ce, ciliated chambers; d, exhalent aperture, or osculum; e, deeper substance of 
the Sponge. 
the flagellate Infusoria in being uniciliated, and the Hy- 
drozoa in having two layers of cells in the body-wall, and 
in producing true eggs. While in other Protozoa aggre- 
gation is a result of growth, and the parts are not mutual- 
ly dependent, in Sponges the parts work for the life of the 
whole, giving the mass a kind of individuality. Differen- 
