BY JAS. P. HILL. 
17 
tubular cavity merges gradually into that of the central blood 
space. Anteriorly, however, the anterior end of the tubular 
infolding projects towards the end of the heart-bladder as a short 
free blind sac which extends into the cavity of the heart-bladder 
beyond the anterior end of the longitudinal slit placing the 
tubular cavity in communication with the proper cavity of the 
central blood space. Consequently in a series of transverse 
section this free end first appears as an apparently isolated cavity 
with muscular walls lying in the cavity of the heart-bladder and 
quite independent of its ventral wall (fig. 3, ivw.). 
The only parallel for this condition in other Enteropneusts 
appears to exist in Balanoylossu* canadensis, which, according to 
Spengel, possesses a similar infolding of the heart-bladder wall, 
although it is still more complicated in other respects. 
According to Spengel the ventral wall of the heart-bladder 
serves to furnish the central blood space with the musculature by 
whose contraction the blood is forced out of the central blood 
space into the sinuses of the glomerulus, and thence into the 
efferent proboscis vessels. We have therefore to regard this 
infolding of the ventral heart-bladder wall into a tube free 
anteriorly, inasmuch as it increases the power of that wall, as a 
special modification to ensure the better performance of its pro- 
pelling function. I have found this condition so frequently in 
this species that it may be taken as a character of specific value. 
In the interior of the heart-bladder a space exists in its greater 
extent (figs. 3, 4, 5 and 14, h), but at its anterior and posterior 
ends the cavity is obliterated by a cellular tissue. Anteriorly 
this tissue has the appearance of a spongy connective tissue with 
numerous nuclei in its connecting strands, and I have not been 
able to observe in this any distinction into two portions, one 
derived from the dorsal, the other from the ventral wall, such as 
Spengel describes for Pt. minuta. An irregular cavity appears 
in the dorsal part of this mass, a short distance behind the anterior 
end of the heart-bladder, and the loose tissue below it gradually 
becomes reduced in size passing into an irregular layer of 
endothelial cells on the ventral wall of the heart-bladder. These 
B 
