NOTES ON THE HISTEIOBDELLID.E. 
203 
each side the brandies to tlie dorsal fibres are given off from 
an almost vertical lamina, througli the ventral part of which 
the nephridium runs. Enclosed between these two laminae 
on the ventral side of the intestine is a longitudinal channel, 
which forms the chief part of the body-cavity in this region. 
Further forwards, towards the anterior limit of the caudal 
region, the laminae of the coelenchyme become exceedingly 
thin, and the branches passing to the dorsal series of 
muscular fibres appear as offsets from the splanchnic layer, 
those to the ventral fibres being, as before, given off from 
the main longitudinal strand in which the nephridium runs. 
At the point where the tail passes into the trunk the 
relations of the ccelenchyme and the muscular fibres become 
somewhat complicated owing to the development of an 
oblique septum, partly muscular, dividing the body-cavity in 
this position ; and in the reproductive segments in front of 
this a similar degree of complication is brought about in the 
female by the presence of the large ripe ovum, and in the male 
by that of the penis, accessory glands and vesiculae seminales. 
In the male in the posterior part of the genital region the 
most considerable part of the coelenchyme takes the form of 
a pair of wing-like, nearly horizontal, lateral plates projecting 
out from the splanchnic layer covering the intestine, and 
giving off branches to the dorsal muscular fibres. The 
coelenchyme does not here form a continuous somatic layer 
on either surface. Further forwards there are formed a pair 
of vertical partitions, partly muscular, separating off a 
median chamber containing the intestine, the nerve-cord, 
the penis and a median part of the testis, from two lateral 
chambers enclosing the lateral portions of the testes, the 
vesiculge seminales and the prostate glands. In this part the 
coelenchyme only gives rise locally and imperfectly to a 
somatic layer. The splanchnic layer is reflected over the 
testes, etc. 
In the female the posterior part of the genital region is so 
distended by the enormous ripe ovum (fig. 10) that the other 
parts become reduced and compressed, and the coelenchyme 
