146 GEORGE W. TANNREUTHER 
with the exception of the very short stomodaeum and procto- 
daeum is derived from the entomeres. The proctodaeum is 
on the dorsal side of the tenth segment. The stomodaeum is 
formed at the apical pole. The embryo is completely turned on 
itself, 1.e., the extreme anterior and posterior ends are in Immedi- 
ate contact. The outer or curved surface, becomes the ventral 
side of the future adult worm. 
NATURAL HISTORY 
The Discodrilids occupy rather a unique position in the annelid 
group. They resemble the Hirudinea in their parasitism, in 
their general shape, in the presence of an anterior and posterior 
sucker and in the existence of chitinous Jaws. The last char- 
acter is not found in any other oligochaet, but occurs in a large 
number of leeches. These facts, perhaps not important in them- 
selves, are indications of a very close relationship between the 
Discodrilids and the Hirudinea, a group which they approach, 
not merely in such habits as the formation of the cocoon in which 
the eggs are enclosed, but in many other points of internal and 
external structures. The fundamental differences between the 
two groups are not numerous and are not of such importance 
as has been assigned them by different writers. The Discodrilids 
are classified as a distinct family of the Oligochaeta. 
Bdellodrilus philadelphicus occurs very abundantly on Cam- 
barus virilis, especially in the early spring and summer months. 
A few may persist throughout the entire winter in their natural 
habitat. 
For convenience, the animal may be divided into three dis- 
tinct regions; the head (pharynx), the body proper, and the 
postericr sucker. The head is much broader than the anterior 
body segments. The head is composed of four distinct annuli, 
which perhaps represent distinct segments. The first or peri- 
stomal annulus is divided into very mobile dorsal and ventral 
lobes or lips, which exhibit slight median emarginations, but 
are otherwise entire. It has sensory hairs and papillae, which 
are common in this family. The fourth annulus is very narrow. 
