AND DEVELOPMENT OF PYROSOMA. 231 
If a line traversing the centre of the cyathozooid and the centre of the ovisac be 
regarded as the axis of the whole foetus, then, in the present condition, the longest 
diameters of the first and of the last ascidiozooids are parallel with that axis, and that 
extremity of each, at which the elaeoblast is situated, is directed away from the eyatho- 
zooid. The long diameters of the intermediate ascidiozooids, on the other hand, cut the 
axis of the foetus at a high angle, their elaeoblastie ends being those which are nearer the 
cyathozooid (fig. 13). 
As development advances, the first and the fourth ascidiozooids retain their parallelism 
to the axis of the foetus, while the whole series elongates, so that the fourth comes to be 
situated close to the first (Plate XXXI. fig. 14), the four encircling the base of the cyatho- 
zooid completely. This elongation of the whole series is effected, mainly, at the expense 
of the isthmuses, which elongate so much as to be converted into slender cords, of which 
the first connects the cyathozooid with the neural face of the first acidiozooid ; the second 
connects the haemal region of the first ascidiozooid, at a point just opposite the endostylic 
cone, with the neural face of the second ascidiozooid ; the third similarly unites the second 
and the third ; and the fourth, the third and fourth. 
But the elongation of the isthmuses is not merely sufficient to allow the fourth ascidio- 
zooid to come close to the third ; it is also enough to permit of a movement of rotation on 
the part of the second and third ascidiozooids. The first and fourth, as has been seen, 
early take up such a position that their long axes are parallel with the axis of the foetus ; 
and, by degrees, the second and third revolve, their adjacent ends being allowed to sepa- 
rate by the elongation of their connecting isthmuses, until their long diameters, from being 
very obliquely inclined to that axis, also become parallel with it, and with the long dia- 
meter of the first and fourth. Thus, at last, the long diameters of all four ascidiozooids 
are parallel with one another and with the axis of the foetus, their similar ends being 
turned the same way (fig. 14), while the isthmuses slope obliquely from the neural region 
of one to the haemal res-ion of the next. The Ions' diameter of each ascidiozooid is at i * ' 
n 
angles with its proper axis (which would be a line drawn from the oral to the cloacal aper- 
ture), and, hence, the neural and haemal sides of the body are at opposite ends of its 
long diameter. The neural side is that, which is turned in the same direction as the 
aperture of the cvathozooid, while the haemal side is the opposite. The mouth is at 
that end of the true axis or short diameter of the body which is turned outwards ; while 
the atrial aperture eventually makes its appearance at the other end of this diameter and 
consequently, on that face of the ascidiozooid which is adjacent to the ovisac and 
cyathozooid. . 
At the commencement of the series of changes here indicated the ascidiozooids are, indi 
that it 
virtually, much smaller than the combined cyathozooid and ovisac ; but as development 
advances, the latter diminish while the former increase ; and as, by the increase of size ot 
the ascidiozooids, the interval between them becomes both relatively and absolutely less 
they, at last, completely hide the combined cyathozooid and ovisac fromview 
not alwavs an easy matter to find the latter (Plate XXXI. fig. 15). 
concomitantly with the ascidiozooids, enveloping them and filling up their inter ™ ls s0 * 
finally, to form a spheroidal investment for the entire tetrazooidal foetus (figs. 14 & 15) 
The test 
