430 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
muscles of the trabeculae. Processes from the connective tissue layer 
pass in among the muscies of the wall, and, uniting again, form an 
inner layer to which the muscles of the trabecule are attached. The 
fasciculi of the latter resemble those of the atrium, but are placed 
more closely together. Their arrangement leaves a central cavity 
which extends from behind the atro-ventricular opening to the bul- 
bus, and many smaller spaces as well. The surface of the ventricle 
is smooth, and between the two sets of muscles there are no lymph- 
spaces as described by Kasem-Beck and J. Dogiel' in their investiga- 
tions on the heart of Hsou and Acipenser. There are large spaces in 
the inner connective tissue layer toward the apex, opposite the osttum 
atro-ventriculare, but they are blood-cavities connected with the 
other spaces of the ventricle. While I have not attempted to demon- 
strate the endothelial layers described by the above-mentioned inves- 
tigators, I doubt the existence of the inner one in Amiwrus, for at 
points the muscle-fibres of the one layer pass into the other as do also 
the connective tissue fibres, except at the spaces. In comparing the 
structure of the ventricle with that of the atrium the only difference 
is that the former has 4 dense muscular layer without blood-spaces 
developed between the connective tissue layer and the trabecule carne, 
which greatly strengthens the wall. The heart of such fishes as are 
supposed to possess double walls should be further studied, and 
especially its development. 
The base of the badbus is provided with a narrow neck which is 
inserted into the central cavity of the ventricle to which it is attached 
by its outer surface. At this opening a pair of valves is attached. 
to the muscles of the ventricle similar to those attached to the atrium. 
Their extremities, however, extend forward as ridges upon the wall 
of the bulbus to strengthen them. Curving upward the bulbus 
passes into the ¢runcus arteriosus, which runs almost at right angles. 
to the axis of the ventricle. 
The walls of the bulbus, ventricle and atrium are well supplied with 
blood-vessels. An artery passes along the dorsal surface of the bul- 
bus to the ventricle ; where it divides in two stems which distribute: 
themselves on each lateral surface. Another artery runs along the 
ventral surface of the bulbus and ventricle and gives offa branch on 
the former to the dorsal surface of the latter. The veins pass back-. 
1 Beitrag zur Kennt. d. Structur u. Function d. Herzen d. Knochen-fische, Zeit. fiir wiss. Zoo- 
Vol, XXXVITI., p. 247, 
