BY JAS. P. HILL. 17 



tubular cavit}'^ merges gradually into that of the central blood 

 space. Anteriorl}^, however, the anterior end of the tubular 

 infolding projects towards the end of the heart-bladder as a short 

 free blind sac Avhicli 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, ivio.). 



The onl}^ parallel for this condition in other Enteropneusts 

 appears to exist in Balanof/Jossus 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. iiiinuta. An irregular cavity appears 

 in the dorsal part of this mass, a short distance l)ehind the anterior 

 end of the heart-bladder, and the loose tissue l^elow it gradually 

 becomes reduced in size passing into an irregular layer of 

 endothelial cells on the ventral wall of the heart-ljladder. These 



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