ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA — 335 
act together, to shorten and narrow the ascidiarium. I do not suppose 
that their effect in the latter direction can be very great; but it might 
well be sufficient to account for the slight contraction of the whole 
ascidiarium, and consequent retrogressive motion, observed by Péron 
and others. 
In my previous memoir, I have pointed out that the round, 
granular, yellowish patches on each side of the entrance of the 
branchial sac, and opposite the middle of the peripharyngeal ridge, 
are not, as Savigny imagined, the ovaries. I am greatly inclined to 
regard them as renal organs, but for the present defer the discussion 
of their structure and functions, 
The reproductive organs of each ascidiozooid of Pyrosoma may be 
divided into actual and potential—the genztalia of the ascidiozooid 
itself, and the blastema whence the genitalia of its buds will take their 
origin. I shall call this last the generative blastema; while the 
genitalia proper are divisible, as I have already pointed out, into a 
single ovisac and a single testis. Both ovisac and testis are situated 
in the left five-sixths of the roof of the mid-atrium; but the testis is 
to the right, the ovisac to the left. The size of both these organs has 
a definite relation to the age and advancement in development of the 
ascidiozooid in which they occur, being larger and more advanced as 
it is older and nearer perfection. Hence the early stages of both 
ovisac and testis can only be observed in buds and young ascidio- 
zooids. The generative blastema will be most conveniently considered 
in connexion with the process of gemmation and in describing the 
most advanced condition of the foetus. The description of the ovisac 
will form the most fitting commencement of the history of sexual 
propagation in Pyrosoma (§ 3); all that remains, therefore, is to give 
in this place some account of the structure of the testis and of the 
character of its products. 
The testis lies in the hemal sinus above the mid-atrium, and on 
the right side of the ovisac, It consists of about a dozen cylindroidal 
ceca, free at their neural ends, but connected at their haemal 
extremity with the dilated upper end of a vas deferens, which passes. 
directly to the neural side and somewhat backwards, to open by a 
slightly raised papilla on the roof of the mid-atrium. The ceca are 
;th of an inch long, or thereabouts. Each consists of a delicate 
structureless membrana propria investing an aggregation of spheroidal 
corpuscles about ;g55th of an inch in diameter. Near the attached 
end of each cecum the rod-like heads of the spermatozoa become 
visible, and gradually take the place of the spheroidal cells, The 
duct has the same structure as the ceca. It presents an upper and 
