ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 371 
the neural wall of the cavity, just above the tubercle of the 
ganglion. 
The urinary (?) organ is very distinct as a mass of pale, sphe- 
roidal granular bodies, and occupies its normal place. 
The ganglion, and so much as could be made out of the ciliated 
sac are similar to the same structures in adults ; but the ganglion has 
a length of only z1 9th of an inch. 
In one of the ascidiozooids of this specimen the isthmus can be 
well studied as it passes off from the neural side immediately behind 
the ganglion. Where it joins the ascidiozooid it is 4th of an inch 
wide, but, in the middle of its length, it has a diameter of not more 
than sth of an inch. In consequence of its passing obliquely from 
the neural face of one ascidiozooid to the hamal face of the next, it 
is, of course, rather longer than the largest diameter of the ascidio- 
zooid (or more than ;nd of an inch long). Viewed from the side, it 
looks like a clear, transparent tube, divided by a partition into two 
channels; but where it bends round, and so exhibits a transverse 
section, this partition is itself clearly seen not to be a simple septum, 
but to be formed by two membranous lamellae, which stretch from 
wall to wall of the isthmus, and are themselves separated by an 
interval of zJygth of an inch. In fact, the central canal has now 
assumed this partition-like character. If traced up to the neural 
wall of the one ascidiozooid with which it is connected, the outer 
membrane of the isthmus obviously passes into the outer tunic of the 
ascidiozooid, while the walls of its contained, inner canal are con- 
tinuous with the inner tunic, or pharyngeal mucous membrane, of 
the same part. On the other hand, if it be followed to the hemal 
wall of the other ascidiozooid, the outer membrane of the isthmus 
passes into the outer tunic of that region, while the wall of the inner 
tube is continuous with the endostylic cone. It is obvious, therefore, 
that the composition of the isthmus is, in reality, the same as in 
earlier stages and that, while its central canal connects the pharyn- 
geal cavities of the two ascidiozooids, the interspace between this 
canal and the outer walls of the isthmus connects their sinuses. 
Between the attachment of the isthmus and the cesophageal 
aperture only two languets are developed from the hypopharyngeal 
region. The great sinus beneath them is full of agglomerated blood- 
corpuscles. 
The endostyle is still broad proportionally (z$5th of an inch), but 
all its parts are well developed. It ends posteriorly in a short pro- 
cess or endostylic cone, z35th of an inch long, which, as I have said 
above, passes into the central tube of the isthmus. 
BB 2 
