4 
370 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
apertures, now 735th of an inch in diameter, are still fully 515th of 
an inch below the margin of the cyathozooid; and although the 
formation of the true cloacal chamber has commenced by the separa- 
tion of the test from its cyathozooidal mould, yet its depth is so 
slight (not more than ;3,th of an inch) that the end of the tongue- 
like inward prolongation of the test still lies between the lips of the 
mouth of the cyathozooid. 
A foetus of Jsth of an inch in diameter has the combined cyatho- 
zooid and ovisac not more than J,th of an inch long, and cup- 
shaped—its upper, open end being as broad as its middle. The atrial 
apertures of the ascidiozooids (which measure ;),th of an inch in long 
diameter) are vertically oval, J,th of an inch long, and lie almost 
wholly above the level of the upper edge of the cyathozooid. They 
open at once into the cloacal cavity, which, as measured from its roof, 
formed by the now hardly-projecting tongue-shaped process, to the 
upper edge of the cyathozooid, is j,th of an inch deep. 
The stolons of this foetus are ;4;th of an inch long, and are 
directed towards the aperture of the cloaca. 
In one of the most advanced foetuses I have met with (PI. XXXI. 
[Plate 30] fig. 15), about #;th of an inch in diameter, the greatest 
length of the ascidiozooids (or the diameter parallel to the foetal axis) 
was jnd of an inch, while their antero-posterior diameter was 2,nd 
of aninch. The long diameter of the combined ovisac and cyatho- 
zooid (the latter being now completely hidden between the hamal 
moieties of the ascidiozooids) was only ~,th of an inch ; or, in other 
words, they had nota third of their former dimensions. Each ascidio- 
zooid of this feetus has a roughly semicircular profile, the straight side 
being turned towards the axis of the foetus. The curved contour is 
more convex on the hemal, more flattened upon the neural face. 
From side to side each ascidiozooid is much compressed, so as not 
to measure more than -2gth of an inch in this direction. 
The oral aperture is not yet pervious ; but a circular groove of the 
outer surface of the test, j,,th of aninch in diameter, indicates the area 
in whose centre it will appear, around which centre lie the oral sphincter 
and the tentacular fringe. The latter, at present, not only projects 
into the buccal cavity but is divided into its processes ; and the hemal 
tentacle, ;3,th of an inch long, exhibits its characteristic enlarged 
base and finger-like process. The peripharyngeal ridge exhibits its 
distinctive structure. Rather in front of its upper loop, a small 
process (the upper end of the diapharyngeal band) projects from 
the roof of the pharyngeal sac; and a corresponding remnant of 
the lower end of the same band is seen, as a small projection of 
