J 
ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 363 
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 elzoblast is situated, is directed away from the cyatho- 
zooid. The long diameters of the intermediate ascidiozooids, on the 
other hand, cut the axis of the foetus at a high angle, their eleeoblastic 
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 XX XI. [Plate 30] fig. 14), the four encircling the base of the 
cyathozooid 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 con- 
nects the cyathozooid with the neural face of the first ascidiozooid ; 
the second connects the hzemal 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 ascidiozooid 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 separate by the elonga- 
tion 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 diameter 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 hemal region of the next. The long 
diameter of each ascidiozooid is at right angles with its proper 
axis (which would be a line drawn from the oral to the cloacal 
aperture), and, hence, the neural and hemal 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 cyathozooid, 
while the hemal 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 
