The Morphology of a Solenogastre. yall, 
other branches pass from the aorta between the halves of the 
reproductive gland but owing to the absence of corpuscles it is 
impossible to trace their course. Having arrived at the front end 
of the gonad the aorta, gradually increasing in size, passes along 
the dorsal wall of the stomach and opens into the head cavity (Fig. 3). 
Attention has already been directed to the fact that the head 
cavity is separated from the visceral sinus by a well defined septum 
perforated only by the aorta and pedal sinus. While the first of 
these vessels terminates with the septum the pedal sinus, having 
passed through the relatively small opening of the diaphragm and 
again expanded, extends forward along the ventral side of the body 
to a position a short distance behind the front end of the radula 
supports in Fig. 3. From this point forward it rapidly diminishes 
in size and communicates with the head cavity by numerous openings. 
It is a most interesting fact that the pedal sinus behaves as 
though the foot persisted. 
As is well known the anterior diaphragm in the chitons is in 
contact with the hinder end of the radula, is opposite the front end 
of the foot and the pedal sinus and aorta do not extend beyond it 
anteriorly. Now if the chitons and solenogastres have been derived 
from a common stock, as appears to be the case, the relations of 
the pedal sinus and diaphragm in the present case may be explained 
by the assumption that the radular apparatus increased in size 
carrying the septum backward; that since the foot persisted in its 
original relations and disappeared at a considerably later date it 
and its attendant vessels were not implicated in the shifting. 
From Figs. 5, 10 it will be seen that while the pedal sinus 
continues to hold a mid ventral position and so pursues its course 
to the hinder end of the body it is accompanied on each side by a 
lateral sinus wide enough to include the pedal and generally the 
pallial cord. The lateral sinuses contain irregular clumps of relatively 
large spongy cells sometimes held together by delicate connective 
tissue fibres which in the neighborhood of the pedal sinus are quite 
compact and effectually prevent the escape of blood except through 
definite sinuses of irregular course that more laterally become ill 
defined. The cells composing the groups are irregular in form, 
highly granular in appearance and contain large spherical activly 
sraining nuclei. In general appearance they resemble the concrement 
bearing plasma cells described by Brock (1883) for certain gastropods 
or the elements that occur in great numbers in the foot and mantle 
46* 
