AND DEVELOPMENT OF PYROSOMA. 237 



projects from the roof of tlie 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 -g-ioth 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 ascidio- 

 zooid it is -^fsth of an inch wide, but, in the middle of its length, it has a diameter of not 

 more than g loth 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 -^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 

 :j^ijPoth 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 it is connected, 

 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 hj^oopharyngeal region. The great sinus beneath them is full of 

 agglomerated blood-corpuscles. 



The endostyle is still broad proportionally (T5~oth of an inch), but all its parts are well 

 developed. It ends posteriorly in a short process or endostylic cone, -sio'tli of an inch 

 long, which, as I have said above, passes into the central tube of the isthmus. 



A cellular mass, 5^i:oth 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, T 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, Aihich is remarkable when one considers the early 

 appearance of the ovisac in the bu.ds. 



The branchial stigmata are altogether tvfelve 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- 

 chial bars are nine in number. The intestine has nearly the same form as in the adult, 



