AN ICHTHYOBDELLID PARASITIC ON SAND WHITINC. 17 
its extension it contains the ventral nerve cord and the ventral 
blood-vesssl. 
Anteriorly it is considerably expanded and entirely sur- 
rounds the proboscis and related organs. 
A considerable expansion of the lacuna also contains the 
ovaries, and again it is dilated in the region of the posterior 
ganglionic mass. A pair of segmental communications is given 
off at the level of each nerve ganglion, joining with those given 
off more posteriorly in each segment by the dorsal lacuna. 
Everywhere the ventral lacuna is lined by a membrane of 
the same character as that of the dorsal lacuna, but the 
muscle-fibres are few. 
The Segmental Lacunae, as already mentioned, extend 
from the dorsal and ventral lacunae. They unite towards the 
lateral region of the body, just past the testes of each seg- 
ment, and are also found in the thick-walled intestinal region. 
Reference to the figure of the Ccelomic System (PI. 2, 
fig. 8) will aid the explanation of the course and branching of 
the segmental lacunae. 
Immediately after the junction of the dorsal and ventral 
extension, the lacuna divides and more laterally each division 
again divides in two ; of the four ultimate branches, the two 
inside ones unite and open into the contractile lacuna (PI. 2, 
figs. 8, 9, s. q. 
The two outside branches unite with the outside branches 
of the preceding and succeeding segmental lacunae respectively. 
Thus it follows that in each segment there are two openings 
of the segmental lacunae into the contractile lacuna. These 
openings into the contractile lacuna are furnished with muscle- 
fibres of annular arrangement and sphincter action. 
The lining of the segmental lacunae is a continuation of 
that of the dorsal and ventral lacunae ; no muscle fibres are 
found in their walls. 
Contractile Lacunae. — These extend on either side from 
the level of the base of the proboscis to the level of the 
anus, but are only contractile from the beginning of the 
testicular region. As already stated, their chief feature is their 
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