ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA = 331 
Of these stigmata there are about thirty on each side. The most 
anterior and the most posterior ones are shorter than the others. 
Anteriorly, in fact, the first is not more than one-third or one-fourth, 
or even less, as long as the vertical height of the branchial sac. The 
stigmata, however, increase in length up to the sixth, and then 
acquire nearly the height of the sac, so as to leave only a small 
imperforate space on each side of the languets, on the neural side, 
and of the endostyle, on the hamal side. Posteriorly the last four 
or five also gradually diminish, until the hindmost of all is not 
larger than the foremost. 
The vertical bars bounding the stigmata are fringed by a single 
series of elongated corpuscles, each of which bears a row of long cilia, 
and (in the dead state, at any rate) all these cilia project outwards 
into the lateral atrial. 
The branchial stigmata just described are subdivided into quadrate 
meshes by some fifteen longitudinal bars which lie altogether on the 
inner side of the vertical ones, to which they are attached by their 
outer edges, projecting like so many narrow shelves into the pharyn- 
geal cavity, and, as I observed in my earlier memoir on this animal , 
are devoid of cilia. They terminate abruptly at their anterior and 
posterior ends, and they do not exhibit the small denticulations along 
their free edges which I have described in P. atlanticuim. The water 
taken in by the oral aperture must pass with perfect ease through 
these meshes, and then, impelled by the cilia on the vertical branchial 
bars, make its way through the lateral atria and, on each side of the 
intestine, to the mid-atrium, whence it finds an exit by the atrial 
aperture. 
As to the proper digestive canal, the wide aperture of the 
cesophagus lies at the posterior, neural, angle of the pharyngo- 
branchial sac, and has an irregular figure; but whether this 
irregularity is normal, or arises from the collapse of its walls after 
death, I cannot say. The cesophagus narrows as it passes back, and 
then curves sharply round towards the hemal side, to open, after a 
very short course, into the large oval stomach, which lies immediately 
behind the middle of the branchial sac, invested, everywhere but in 
front, by the atrial tunic, and bathed in the blood which lies between 
it and that tunic. At its pyloric end it gives rise to the narrow 
commencement of the intestine, which, after suddenly dilating and 
turning forwards and to the hemal side, bends back sharply upon itself, 
1 In the living condition, as Milne-Edwards has hinted, and as I have shown in my 
memoir (2. c. p. 583), the cilia upon opposite sides of a branchial stigma move in opposite 
dire tions. 
2 Loc. cit. pl. 17, fig. 3, and p. 583, line 4, where the word ‘sinus’ should be ‘ ones.’ 
