AND DEVELOPMENT OF PYROSOMA. 
237 
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 the neural wall of the cavity, just 
above the tubercle of the ganglion. 
The urinary (?) organ is very distinct as a mass of pale, spheroidal, 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 2^0 th of an inch. 
In one of the ascidiozooids of this specimen tbe isthmus can be well studied as it passes 
off from the neural side immediately behind the ganglion. Where it joins the ascitlio- 
zooid it is ^yjth of an inch wide, but, in the middle of its length, it has a diameter of not 
more than iroth of an inch. In consequence of its passing obliquely from the neural 
face of one ascidiozooid to the haemal face of the next, it is, of course, rather longer than 
the largest diameter of the ascidiozooid (or more than aV 11 ^ 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 lamella*, which 
stretch from wall to wall of the isthmus, and are themselves separated by an interval of 
th of an inch. In fact, the central canal has now assumed this partition-like charac- 
ter. If traced up to the neural wall of the one ascidiozooid with which 
the outer membrane of the isthmus obviously passes into the outer tunic of the ascidio- 
zooid, while the walls of its contained, inner canal are continuous with the inner tunic, or 
pharyngeal mucous membrane, of the same part. On the other hand, if it be followed to 
the haemal 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 
pharyngeal 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 oesophageal 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 ( T f -th of an inch), but all its parts are well 
developed. It ends posteriorly in a short process or endostylic cone, ^th of an inch 
long, which, as I have said above, passes into the central tube of the isthmus. 
A cellular mass, ^th of an inch long, is attached to the external tunic, close to the end 
of the endostylic cone, if not directly connected with it ; and this, I am inclined to think, is 
the rudiment of the generative blastema. I have not been able to detect any distinct 
structure (as of an ovisac or testis) in it, which is remarkable when one considers the early 
appearance of the ovisac in the buds. % .'""";'"'.'• 
The branchial stigmata are altogether twelve in number. The anterior and posterior 
are rudimentary while most of the others extend across almost the whole depth of the 
branchial sac. The cilia are perfectly distinct upon their edges. The longitudinal bran- 
dual bars are nine in number. The intestine has nearly the same form as in the adult, 
