SCIENTIFIC SUMMARY. 
221 
of the most striking illustrations of the value of the knowledge of the early 
history of an organism is afforded by the embryology of the sponge. 
Haeckel's discovery that the larva sponge is a planula, though not homo- 
logous with the embryo polype or jelly-fish, enables the naturalist to at 
once decide that the sponge is not a Protozoan, but belongs to a type only 
less highly organised than the lower polypes, and with more analogy to the 
Radiates than the Protozoa. If, under the guidance of the results of the 
studies of Lieberkiihn, Carter, Clark, and particularly of Haeckel and 
Metschnikoff, w T e examine the structure of a sponge, we shall find that in 
its simplest form it is a hollow, vertical cylinder, fastened by its base, -with 
the mouth opening upwards from a central gastro-vascular cavity, with 
ciliated epithelial cells lining the cavity, and possessing a surprising degree 
of individuality. There usually are several mouths, and the cavity usually 
opens into a labyrinth of chambers connected by passages through the 
cellular tissue ; these round chambers being lined with ciliated epithelial 
cells. This body is supported by a basket-work of interlaced needles of 
silica or lime, developed in the inner layer of cells of the larva. Such, in 
brief, is the sponge. Does the fact that in the simplest, immature forms, 
we have quite a regular body-wall and a single cavity, compel us to range 
the sponges side by side, and in the same natural division with the polypes 
and jelly-fishes, in the typical forms of which the central cavity acts as a 
mouth ? Metschnikoff has shown that it would seem to be a violation of 
the existing principles of classification to place together animals so unlike. 
The sponges apparently represent a class lower than, but possibly equivalent,, 
systematically, to the polypes and jelly-fishes. 
Professor Huxley's Classification of the Animal Kingdom . — In a paper which 
he read before the Linnean Society in December last, and which was for a 
time withdrawn, so that we had not the opportunity of laying its result 
before our readers in the last number, Professor Huxley makes the follow- 
ing remarks : — “ Animals are primarily divisible into those in which the 
body is not differentiated into histogenetic cells (Protozoa), and those in 
which the body becomes differentiated into such cells (Metazoa of 
Haeckel). I. The Protozoa are again divisible into two groups: 1, the 
Monera (Haeckel), in which the body contains no nucleus ; and 2, the 
Endoplastica, in which the body contains one or more nuclei. Among these 
the Infusoria, Ciliata, and Flagellata (e.g. Noctiluca), while not forsaking 
the general type of the single cell, attain a considerable complexity of 
organisation, presenting a parallel to what happens among the unicellular 
Fungi and Algae (e.g. Mucor, Yaucheria, Caulerpa). II. The Metazoa are 
distinguishable, in the first place, into those which develop an alimentary 
cavity — a process which is accompanied by the differentiation of the body- 
wall into, at fewest, two layers, an epiblast and a hypoblast (Gastrcece of 
Haeckel), and those in which no alimentary cavity is ever formed. Among 
the Gastraeae there are some in which the gastrula, or primitive sac with 
a double wall open at one end, retains this primitive opening throughout, 
life as the egestive aperture ; numerous ingestive apertures being developed 
in the lateral walls of the gastrula — whence these may be termed Polysto- 
mata. This group comprehends the Spongida or Porifera. All other 
Gastraeae are Monostomata, that is to say the gastrula develops but one 
