78 HUXLEY, ON CLASSIFICATION, 
ciliated germs (D, fig. 3), which make their way out, and, after 
swimming about for a while, settle themselves down and grow up into 
Spongilla. 
Now that we know the whole cycle of the life of the sponges, and 
the characters which may be demonstrated to be common to the 
whole of this important and remarkable class, I do not think any 
one who is acquainted with the organization or the functions of 
plants, will be inclined to admit that the Spongida have the slightest 
real affinity with any division of the vegetable kingdom. 
The next group to be considered is the division of the INFUSORIA ; 
and here, again, within the last few years, prodigious strides have 
been made in our knowledge of the subject. Although the Infusoria 
have been favorite studies for many years, still it is only quite 
recently that the cycle of life of these animals has been made almost 
completely known, and that we have become acquainted with the 
true sexual process as it occurs in them 
The different species of the genus Paramecium are very common 
among the microscopic inhabitants of our fresh waters, swimming 
about by means of the vibratile cilia with which the whole surface of 
their bodies is covered; and the structure which essentially charac- 
terises these animals is probably that which is common to the whole 
of the Infusoria, so that an account of the leading structural features 
of Paramecium is, in effect, a definition of those of the group. 
Imagine a delicate, slipper-shaped body inclosed within a structure- 
less membrane, or cuticula, which is formed as an excretion upon its 
outer surface. At one point (Fig. 4, B a) the body exhibits a slight 
depression, leading into a sort of little funnel (b c) coated by a con- 
tinuation of the same cuticular investment, which stops short at the 
bottom of the funnel. The whole of the bag formed by the cuticula 
is lined by a soft layer of gelatinous matter, or “sarcode,” which is 
called the “ cortical” layer (Fig. 4, A a); while inside that, and 
passing into it quite gradually, there being no sharp line of demarca- 
tion between the two, is a semi-fluid substance, which occupies the 
whole of the central region of the body. Neither in the cuticle, the 
cortical layer, nor the central substance, has any anatomist yet dis- 
covered a differentiation into cellular layers, nor any trace of that 
histological composition which we meet with in the tissues of the 
higher animals; so that here is another case of complex vital phe- 
nomena proceeding from a substance which, in a histological sense, 
is structureless. 
At two points of the body (Fig. 4, A ec) the substance of the 
cortical layer exhibits a remarkable power of contraction and dilata- 
tion. If you watch one of those points, the sarcode suddenly seems 
to open like a window and, for a while, a clear space is visible, which 
= ae a 
