EMBRYOLOGY OF BDELLODRILUS 147 
The middle two appear as muscular rings. The chitinous jaws 
are triangular, the dorsal with a single tooth, the ventral jaw 
with a pair of smaller teeth. No lateral mucous glands which 
are very common in some of the species are present. 
The body proper consists of eight strongly bi-annulate somites 
or rings. The anterior somites are longer and broader than 
the posterior. When contracted, the minor annuli of the somites 
are telescoped within the major annuli. The fifth, sixth, and 
seventh somites are sexual. The first, second, third, fourth and 
eighth somites are nephridial. The spermatheca is broad, thin 
walled, and nearly cylindrical. The penis is carried to the 
exterior by the eversible bursa, into which its projecting end 
is received. There is a conspicuous prostate in addition to 
the large glandular sperm sac. These parasitic forms remain 
attached to the ventral surface of the host throughout their 
entire life history. The eggs are deposited on the ventral sur- 
face of the host, more abundantly where the water is kept in 
constant motion by the movement of the appendages. Each 
egg is enclosed in a distinct separate stalked cocoon. The 
base of the stalk is firmly attached to the host. The deposition 
of eggs occurs during the entire year, if the parasites be kept 
in aquaria at room temperature. In their natural habitat 
eggs are not deposited during the severe winter months. 
BRIEF OUTLINE OF DEVELOPMENT 
The cleavage of the ovum takes place with considerable pre- 
cision and regularity. Especially is one impressed with this 
striking phenomenon, after following the cleavage of many 
ova. The only perceptible variations being (a) slight differ- 
ences in the time at which the individual cells divide; (b) slight 
variations in the size of the same cells in different ova. The 
rate of cleavage varies somewhat with temperature. Occasion- 
ally all the cleavage cells of an individual egg are nearly equal 
and it is impossible to orient the embryo before the germ bands 
begin their formation. This, however, is an exception, rather 
than a usual occurrence. 
