PROGRESS OF MICROSCOPICAL SCIENCE. 
25 
cular tissue of the body, and in many ways resembling the cardiac 
tissue of vertebrates. The walls are thickly studded with nuclei, some 
of which possibly belong to an external tesselated epithelium. Other 
nuclei are undoubtedly the proper nuclei of the contractile elements, 
and the remainder seem to be of the nature of connective tissue. 
Of none of them can it safely be said that they are the nuclei of 
nerve-cells. Molluscan nerve-fibres might undoubtedly, unlike ver- 
tebrate medullated nerve-fibres, easily escape detection ; but Mr. A. 
S. Lea, of Trinity College, carefully examined for us the whole of 
both the auricle and ventricle without discovering any distinct 
nervous structures. He also went systematically over the margins of 
both the aortic and pulmonary orifices, but could find no nerves 
running into or out of the heart. In no other way could nerves be- 
come connected with the heart. And, opposed as it may seem to 
general experience, and still more to recognized opinions, we are led 
to the conclusion that the heart of the snail has no nervous connection 
with the rest of the body ; nay, more, that it has within itself no 
distinctly specialized nervous mechanism, but that its contractile ele- 
ments are composed of protoplasm, arranged, it is true, more or less 
in fibres, yet otherwise but slightly advanced in differentiation. 
Structure and Motion of the Spermatozoa . — These have been tho- 
roughly investigated by Dr. T. H. Einer, who has published an im- 
portant paper on the subject. He says : — The spermatozoa of dwarf 
flying-mice show peculiarities which have also been observed in other 
mammalia. The head and centre of these spermatozoa are unusually 
broad, and are not continuous, but are interrupted by a very fine 
thread, which is sometimes quite long, so that this thread can for 
convenience bo called the throat. In the night-flying mouse this 
thread is *0007 of a millimeter in length. In those cases where there 
is no throat to be seen, there is at least a division in the body, and 
we can easily suppose ono to exist, but our lenses are not powerful 
enough to reveal it. A small line is seen in the middle of the centro, 
continuing through the throat into the head and anterior extremity. 
Sometimes this fine line can be seen as a separate line as it passes 
through the throat-piece. 
On the spermatozoa of Vesperugo pepestrellus, cross-lines are often 
seen upon the tliroat-piece, dividing it into three or four square pieces, 
held together by the line central thread. On others of the same indi- 
vidual, this appearance is not seen. The addition of improper fluids 
arrests the motion of the spermatozoa, and then these peculiar mark- 
ings are no longer seen. In other species of mammalia, something of 
the above structure could be found, viz. in horses, mice, and guinea- 
pigs, oftener in cattle and Erminea mustela ; also sometimes in the 
dog and cat. 
In man he has often failed to find the line in the middle piece, 
but the fine joints were frequently made out in the throat-piece. 
The author has not seen the central thread in the head or centre- 
piece of the spermatozoa of any mammalian in perfectly natural semen. 
But some observations on not quite fresh semen seemed to show 
that the central thread in these parts was masked in the fresh state 
