16 
ON A NEW SPECIES OP ENTEROPNEUSTA, 
which have become enclosed during the formation of the skeleton, 
and not cells which have immigrated later. 
" Chondroid tissue": As in the genus Ptychodera generally the 
" chondroid tissue " of the proboscis neck is not greatly developed, 
and the cell strands appearing generally in transverse sections as 
small isolated masses are derived as Spengel has shown mainly 
from the epithelium of the proboscis pockets. As in Pt. clavigera 
a band of " chondroid tissue " continuous with the lateral tissue 
is present between the " end plate " and " keel " of the proboscis 
skeleton. The cell strands of this ventral portion are very richly 
developed, and are derived from the epithelial cells lining the ventral 
proboscis pockets and behind the posterior edge of the proboscis 
septum from the continuation of the same epithelial cells lining 
the ventral unpaired portion of the proboscis coelom. 
Heart-bladder : The heart-bladder is esentially similar in its 
general relations to that of Pt. minuta. It is a completely closed 
sac, having no connection either with the vascular system or with 
the proboscis coelom. On its lateral walls the muscle fibres 
belonging to the dorso-ventral muscle plate are very well marked 
(fig. 5, dsc), but as in the other species of the genus they do not 
possess a musculature of their own. On the ventral wall there is 
present as in the described species a very distinct single layer of 
transverse muscle fibres which, so far as I have observed, are 
entirely confined to this wall. In this species the ventral wall of 
the heart-bladder is infolded into the cavity of the bladder in a 
very characteristic manner. In its posterior part the central 
blood space of the proboscis is a transverse cleft between the 
ventral wall of the heart-bladder and the proboscis gut just as in 
the species previously described, and varies in size according as it 
is filled with blood or empty. In its anterior region, however, 
the ventral wall of the heart-bladder is infolded into the cavity 
of the bladder along the median line so as to give rise to a tubular 
cavity which communicates with the central blood space by a 
narrow longitudinal slit (fig. 4, ivw.). Then posteriorly to the 
infolding by the gradual receding of the two edges of the slit, the 
