1877.] Zoblogy. 179 
simple pavement epithelium. All these cells are placed in juxtaposition 
like the constituent elements of a vegetable tissue. There is no trace 
of a homogeneous layer, of connective tissue, of muscular fibre, of 
nervous elements, nor of intercellular substance. There is only be- 
tween the cells a homogeneous (wnissante) substance, as between epi- 
thelial cells. The axial cell is regarded as homologous with the endo- 
derm of the higher animals (Metazoa). He designates as the ectodermic 
layer the cells surrounding the large, single axial cell. There exists no 
trace of a middle layer of cells. We discover no differentiated appa- 
ratus; all the animal and vegetative functions are accomplished by the 
activity of the ectodermic cells and of the axial cell. On account of 
these characteristics Van Beneden regards these organisms as forming 
the type of a new branch of the animal kingdom which he distinguishes 
as Mesozoa. 
Each species of Dieyema comprises two sorts of individuals differing 
externally, one (the MNematogene) producing vermiform embryos, the 
other (Rhombogene) infusoriform young. “ ematogenes produce 
germs which undergo total segmentation, and assume a gastrula condition. 
After the closure of the blastopore the body elongates, and the worm-like 
form of the adult is finally attained, as they pass through the body-walls 
of the parent. 
The germs of the Rhombogenes arise endogenously in special cells 
lodged in the axial cell and called “ germigenes.” The germ-like cells 
undergo segmentation, and then form small spheres which become infu- 
soriform embryos. The worm-like young is destined to be developed 
and live in the Cephalopod where it has been born, while the infusorian- 
like young probably performs the office of disseminating the species ; it 
transmits the parasite of one Cephalopod to another. 
This work is also an important contribution to histology, particularly 
to the subject of cell-division. Says Van Beneden, “the recent re- 
searches of Auerbach, of Biitschli, of Strasburger, of Hertwich, and 
those that I have published, have established the fact that the division 
of a cellule, that is to say, the multiplication of the cellular individuality, 
is the resultant of a long series of complex phenomena, accomplished 
in a determinate order, and having their seat as much in the nucleus as 
in the substance of the cell.” 
_ Finally, Van Beneden places in his branch of Mesozoa the hypothet- 
ical Gustreeades, which term he applies to (gastrula-like) organisms 
formed of two kinds of cellules, some ectodermic, others endodermic, in 
which the endoderm is formed by invagination. He calls Planulades, 
those hypothetical Mesozoa which are formed from a many-celled sphere 
_ Constituted like a Magosphera (Haeckel) and in which the two cellular 
layers are developed by delamination. He therefore divides the animal 
kingdom into three primary groups, that is, the Protozoa, the Mesozoa, 
and the Metazoa. 
