238 PROFESSOR HUXLEY ON THE ANATOMY 
and the tubular, hepatic system is well formed. The heart is visible in its place. The 
elaeoblast is a mass of clear reticulated tissue, causing the haemal wall to bulge a little on 
each side of the middle line, and occupying the interval between the endostyle and gene- 
rative blastema, on the one hand, and the heart and intestine on the other. 
The atrial aperture is enormous in proportion, occupying the greater part of the inner 
face of the ascidiozooid above the level of the cyathozooid and attaining a length of fully 
And of an inch and a breadth of r^th of an inch. In other words, the atrial apertur 
six times as large as it is in the adult ascidiozooid, though the latter is at least eight or 
ten times as large as one of the zooids of the foetus under description. In consequence 
of the great proportional size of these oval apertures, whose long diameters are parallel 
with the axes of the foetus, the intervening wall of the cloaca is very narrow. 
The cyathozooid and ovisac are -jfeth of an inch long by ^ 6 ih wide and more cylindrical 
than cup- shaped. The aperture, still distinctly visible, has a diameter of 500 th of an inch ; 
and as the cloacal chamber is now ^th of an inch deep, the margin of the aperture is 
but just on a level with the convex, neural margin of the oesophagus of any of the asci- 
diozooids. "Where the former tongue-like process existed, the roof of the cloaca now 
hardly projects inwards at all. 
The atrial muscles are visible as very delicate, straight bands, ^-th of an inch long by 
th wide, which take an oblique course on each side, from a point a little below the 
end of the endostyle, neurad and a little forwards, to a point opposite the commencement 
of the oesophagus. In the middle of their course these bands lie very near the lips of 
the atrial aperture. 
The stolons are ^th of an inch long ; they pass almost horizontally inwards, towards 
the rudimentary lip of the cloaca, and are curved towards its cavity, at their blind extre- 
mities. The corpuscles of which their walls are composed are more elongated than before 
and, sending processes into the adjacent substance of the test, cause the caecal ends ol 
the stolons to have a very peculiar, brushlike appearance. 
3. The Test. — As I propose to reserve the description of the histological changes under- 
gone by the embryo of Pyrosoma for another occasion, I will merely state, in this place, 
that the test appears, at first, to be a structureless excretion. Subsequently, cellular 
bodies, like connective-tissue corpuscles, are discernible in its most superficial layer, an 
are disposed in such a manner as to form a very regular, hexagonal network, with large 
meshes. The most advanced foetus has presented neither of the fibrous layers visible in 
the adult test. 
Ninth Stage. The conversion of the tetrazooidal foetus into the adult ascidiarwni. 
The most advanced foetus which has been described differs from the adult ascidiariuni 
not merely in size, in the paucity of its ascidiozooids, in the form and proportions 
the latter, in the absence of buds, or ever so slightly differentiated reproductive organs, 
in them, and in their large atrial apertures (all of which are peculiarities which we 
may easily conceive to be altered by age and growth), but in still more important charac- 
ters, seeing that in the adult ascidiarium I have met with no trace of the cyathozooid or 
the isthmuses, nor have I been able to discover any ascidiozooid with two stolons. 
