382, ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
‘mamelon’ provided with an orifice” as the homologues of the 
three processes given off from the anterior division of the larval 
body in the simple Ascidians. In this case this ‘mamelon,’ which 
they consider to be the rudiment of the cloaca, must correspond 
with that anterior division. But the examination of their figures 
and descriptions renders it hardly doubtful to my mind that the 
‘mamelon’ is a structure homologous with the cyathozooid of the 
foetal Pyrosoma, the eight rudimentary ascidiozooids of the Botryllus 
being arranged around its base, just as the four are disposed in the 
foetal Pyrosoma. If this reasoning be correct, it follows that the 
cyathozooid of Pyrosoma corresponds with the anterior division of 
the body in the ordinary Ascidian larva, e.g. of Clavelina. 
The peculiar connexion of the embryo Pyrosoma with its ovisac, 
and the extrusion of the latter combined with the embryo, as a single 
foetus, into the mid-atrium of the parent, are, however, peculiarities 
for which we should in vain seek a parallel among ordinary Ascidians. 
But there is one family of this class, the Se/pe, which resemble Pyro- 
soma in having an eleoblast, and in possessing no caudal appendage 
in the larval state (differing in the same respects from all other 
Ascidians), in which the search for analogies is more hopeful. 
Most Salpa, like Pyrosoma, possess, as Krohn was the first to 
point out, but a single ovisac, connected by a peduncle-like duct with 
the wall of the mid-atrium. Prof. Leuckart, who, with a knowledge 
of all that had been written upon the question, subjected the repro- 
ductive processes of the Sa/p@ to a renewed and very careful scrutiny, 
some years ago? stated (Z. ¢ p. 47) that the ovum of Salpa mucronata 
consists of a granular, tolerably viscid yelk, enclosing a large, vesicular 
germinal vesicle, with a simple germinal spot. No vitelline membrane 
was to be detected,—the only covering of the ovum being the ovisac, 
which is closely applied to the surface of the vitellus, and is lined 
internally with a layer of small nucleated cells. The peduncle of the 
ovisac is a short, narrow duct, which only becomes a little thicker at 
its anterior end and, like the ovisac, is lined by an epithelium. Its. 
anterior end opens into the atrium; and in the vicinity of the 
aperture the inner tunic exhibits an elongated, discoid thickening,. 
in which numerous small nucleated cells, like those in the ovisac and 
its duct, are to be detected. This description, it is obvious, would 
apply equally well to the young ovisac of Pyrosoma. 
It does not appear that the entrance of the spermatozoa into the 
duct of the ovisac has been observed in the Sa/pe. 
I have stated in my Memoir already cited (Phil. Trans. 1851 
1 Zoologische Untersuchungen, Zweites Heft, Salpen u. Verwandte. Giessen, 1854. 
