312 
WILLIAM F. ALLEN. 
of considerable size, situated side by side, near tlie tip of 
the tail. They are nothing more than pi-onounced swellings 
of the anterior portion of the right and left forks of the 
caudal veins, which completely cover the lateral surfaces of 
the anterior third of the median ventral cartilaginous bar. 
At the point where the caudal veins (anterior portions of the 
caudal hearts) unite to form the common caudal vein their 
walls extend into the posterior end of the vein as valves, thus 
preventing a back flow of blood or lymph into the caudal 
hearts. 
In the 20 cm. series the greatest caudo-cephalic diameter 
of the caudal heart is 2*52 mm., its greatest dorso-ventral 
diameter is T52 mm., and its greatest median-lateral diameter 
is ‘1 mm. The corresponding measurements in the 85 mm. 
series are T1 mm. by *47 mm. b}^ '1 mm. These measure- 
ments would indicate that the heart is about twice as long as 
it is high. 
Both hearts in section (Fig. 1) have the general form of 
pyramids with their apices pointing dorsad. Usually both 
are filled with i-ed corpuscles, but ordinarily one is distended 
more than the other. If Greene’s observations are correct, 
that tliey contract alternately, it would account for there 
being more blood in one than in the other. 
The inner wall of the heart is endothelium, surrounded by 
a layer of dense white fibrous tissue, which binds it inward 
to the median ventral cartilaginous bar and laterad to the 
musculi cordis caudalis. According to Favaro this layer 
abounds in elastic fibres, but none w’ere visible in my 
sections, although it should be stated that I employed no 
specific stains. The musculi cordis caudalis (Fig. 1, 
M. C. C.), which functions as a partial myocardium, is attached 
anteriorly to the lateral process of the median ventral bar, 
and its stilated longitudinal fibres, after passing posteriorly 
over the lateral surface of the caudal heart, become attached 
to the median ventral bar, and to a slight extent to the bases 
of some of the ventral or anal fin radials. This muscle later 
ou will be shown to be more intimately related to the 
