MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOM^. 
581 
creature is quite freshly taken ; afterwards, the illumination arising from friction is 
only local. 
43. So far as could be observed the Pyrosoma had no power of locomotion ; any 
such power arising from a contraction of the hollow cylinder is out of the question, 
as its substance is cartilaginous and non-contractile. Any one who does not examine 
these animals quite closely may be readily deceived on this point ; for the alternate 
fading and brightening of the phosphorescent light gives rise to the impression that 
the creature recedes from and approaches the eye ; and viewing them from the deck 
of a ship only, it is diflScult to imagine that they do not really move with some 
rapidity*. 
44. The Pyrosoma may be described as a hollow cylinder, solid and hard to the 
touch, closed and rounded at one end, open at the other. A narrow lip projects 
inwards at the open extremity ; it has been called a membranous diaphragm, but in 
the specimen examined it was certainly cartilaginous and immoveable, like the rest 
of the animal. The thickness of the wall of the cylinder was about two-fifths of an 
inch; its diameter was about 1 inch and a half; its length was about 10 inches. 
The outer surface of the cylinder was covered with a multitude of small projections, 
and close to them opened small circular apertures. The inner surface of the cylinder 
was uneven but not rough, and was similarly pierced with circular apertures. 
The wall of the cylinder consists of a vast number of minute Ascidian “ zooids” 
lying perpendicular to the axis of the tube, and united together by a common carti- 
laginous basis ; and the small circular apertures correspond respectively, the outer 
to the anterior aperture of the Salpa, the inner to the posterior aperture. 
Each aperture is provided with a small dentated membranous valve, Plate XVII. 
fig. 1. 
45. In each zooid there is at one point a ganglion {d), with a mass of deep red 
otolithes. As in Salpa, this must be called the ventral side ; the opposite is the dorsal 
side, and contains (as in Salpa) an endostyle, Plate XVII. figs. 1, 2 c. The ganglionic 
or ventral surfaces of all the polypes are turned the same way, and towards the open 
end of the cylinder. 
By far the greater part of the space occupied by each zooid is taken up by the 
respiratory cavity. This is elliptical, and compressed laterally. It is lined by the 
proper branchial network, hereafter to be described (r;), and communicates freely 
by means of the apertures in the branchial network with the post-branchial or anal 
cavity, which, as before stated, opens into the interior of the cylinder. 
46. The viscera lie behind the branchim. They consist of the digestive canal, heart, 
and generative organs. 
* My observations upon the power of locomotion of Pyrosoma were very imperfect, as I was anxious rather 
to attend to the more interesting points of structure. Certainly the cylinder does not contract as a whole, but 
it is very possible that the zooids do, and so move by the reaction of the forced-out water against the closed 
end of the cylinder. 
MUCCCLI. 4 F 
