200 THE MICROSCOPE. 
9 f, Fig. 3, are certain hollow cylinders of problematical importance. 
They extend along the commissures the entire length of the body. 
Over the top and beneath is a sheath (m s) of muscular fibers 
extending lengthwise. Beneath the ganglia and the longitudinal 
commissures runs a blood vessel (s nv). The ganglion is somewhat 
distinctly bilobed; the large nerve cells (” c) are arranged somewhat 
clearly with reference to a median vertical plane. These cells, it 
will be noticed, strongly resemble the unipolar nerve cells of the 
human nervous system. It is not possible to trace distinctly the 
continuity of the processes with the nerve fibers which make up the 
body of the ganglion. L n shows the lateral nerves which extend 
out to the body wall. 
Fig. 4 shows the ovary. A pair of these are found in the 
thirteenth segment attached to the hinder face of the dissepiment, 
between it and the twelfth. The ova at ov, near the small end of 
the pear-shaped mass, are nearly mature. Passing toward the base, 
they are less distinct, until they are finally not to be distinguished 
from the other cells of the ovary. About each ovum is a layer of 
very small cells, which serve to nourish the ovum to the point of 
maturity. The ova are finally discharged into the coelomic cavity, 
and reach the outside by special ducts. 
Fig. 7 represents one of the excretory organs, called nephridia. 
Of these, there is a pair in each segment having an internal opening 
by the funnel (g). This funnel extends through the dissepiment (d) 
into the next anterior segment. This funnel and the tube (c) into 
which it leads, are richly ciliated. The portion g has thicker walls, 
and is lined with a glandular layer of epithelial cells. This 
glandular portion opens into a muscular sac, m, which opens by a 
pore upon the surface of the body. In passing, it is worth noting 
that many of these features are essentially those present in the 
excretory system of higher animals. 
I have in this brief review touched upon a few of many inter- 
esting details which present themselves in the study of these humble 
excavators. They are enough, I trust, to show that there are few 
objects available to the histologist from which a greater number of 
fundamental facts can be demonstrated. 
DESCRIPTION OF THE. FIGURES. 
Fic. 1.—A diagrammatic sketch of a cross-section ise at 
about the fortieth segment: c, structureless cuticle; ¢ p, columnar 
epithelium; c m, circular layer of muscles; / m, longitudinal 
muscles; s, sete; cw, ccelomic cavity; a c, alimentary cavity; 
tph, typhlosole; d v, dorsal blood-vessel; c v, circular blood-vessels, 
