CLASSIFICATION OF THE ANIMAL KINGDOM. 53 
absolutely nothing is positively known respecting the 
phylogeny of any of the larger groups of animals. Valuable 
and important as phylogenic speculations are, as guides to, 
and suggestions of, investigation, they are pure hypotheses 
incapable of any objective test; and there is no little danger 
of introducing confusions into science by mixing up such 
hypotheses with Taxonomy, which should be a precise and 
logical arrangement of verifiable facts. 
The present essay is an attempt to classify the known facts 
of animal structure, including the development of that struc- 
ture, without reference to phylogeny, and, therefore, to form 
a classification of the animal kingdom which will hold good 
however much phylogenic speculations may vary. 
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 (Merazoa of Haeckel). 
I. The Prorozoa are again divisible into two groups: 
1, the Monera (Haeckel, in which the body contains ne 
nucleus; and 2, the Endoplastica, in which the body contains 
one or more nuclei. Among these, the Jnfusoria ciliata and 
flagellata (Noctiluca, e.g.), while not forsaking the general type 
of the single cell, attain a considerable complexity of organisa- 
tion, presenting a parallel to what happens among the 
unicellular Fungi and Alge (e. g., Mucor, Vaucheria, Cau- 
lerpa). 
{I. The Merazoa 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 
(Gastree of Haeckel), and those in which no alimentary 
cavity is ever formed. 
Among the Gastreze, 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 aper- 
ture; numerous ingestive apertures being developed in the 
lateral walls of the gastrula—whence these may be termed 
Polystomata. This group comprehends the Spongida or 
Porvfera. All other Gastrez are Monostomata, that is to say, 
the gastrula develops but one ingestive aperture. The case 
of compound organisms in which new gastrule are produced 
by gemmation is of course not a real exception to this rule. 
In some Monostomata the primitive aperture becomes the 
permanent mouth of the animal (Archzostomata). 
This division includes two groups, the members of each of 
which respectively are very closely allied :—1. The Ceelen- 
