572 
MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
It would appear probable that the languet and the ciliated fossa subserve in some 
manner the performance of the gustatory function. 
17 . From each side of the base of the languet a narrow “^ciliated band” (j?) runs 
upwards, until it meets with its fellow of the opposite side, the two thus encircling the 
anterior aperture of the respiratory cavity. 
18. The dorsal wall of the respiratory cavity is marked by two longitudinal folds, 
running from before backwards to the mouth. These are the dorsal folds of Savigny 
and others; but there is an organ to which the name of “ Endostyle ” may be given (c), 
very distinct from these, and yet which has been invariably confounded with them, 
consisting of a long tubular filament, with very thick strongly refracting walls, 
Plate XV. fig. 4 c. This body lies in the dorsal sinus ; its anterior extremity is slightly 
curved downwards, somewhat pointed, and looks stronger and more developed than 
the posterior extremity, which is paler, more delicate and truncated. By its ventral 
surface this ‘‘endostyle” is attached to a ridge of the inner tunic, which rises up 
into the dorsal sinus. 
19. It has been stated that the circulatory system consists, not of vessels with distinct 
parietes, but of more or less irregular sinuses. However irregular in form, the position 
of several of these is very constant. There is a dorsal sinus running along the dorsal 
surface and enclosing the internal shell ; there is a ventral sinus opposite to this and 
containing the ganglion ; there are lateral sinuses connecting these. Then there is 
the sinus in which the intestine and generative organs lie, the peri-intestinal sinus, 
and, finally, the sinus which, connecting the dorsal and ventral system of sinuses, 
traverses the gill and constitutes the branchial sinus. 
These sinuses all communicate together round the oesophagus, and above and 
in front of this, the heart {g) is developed. The heart lying obliquely at the posterior 
extremity of the dorsal sinus, is not tubular, as it has been described ; it forms 
not more than three-fifths of a tube ; nor is it correct to say that it lies in a pericar- 
dium. Its true nature will be best conceived by supposing the inner surface of a sinus 
to have become developed for about three-fifths of its circumference into a free mus- 
cular membrane, Plate XV. fig. 9. 
This membrane is exceedingly delicate, and is composed of a single layer of flat 
striated muscular fibrils. 
20. The direction of the circulation depends entirely upon the order of contraction 
of the muscular fibrils of the heart. If they contract successively from behind for- 
wards, the blood is forced in that direction ; after a certain number of such contrac- 
tions, they all become simultaneously, as it were, paralysed for a short period, and 
then they begin to contract again, but in the inverse order, and of course with an 
opposite effect upon the direction of the circulation. 
The blood, in its alternate flux and reflux, bathes all the internal organs — the in- 
testine, the endostyle, the brain and the generative organs, the corpuscles finding 
their way as they best may among the interstices. When the force of the heart 
