ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 353 
seems impossible to account for the presence of the dense mass 
of filaments within it, except on the supposition that they have 
an inherent propulsive power ; and the only free, filamentous bodies 
possessed of such a power we know of, in the animal economy, are 
spermatozoa. 
Furthermore, in my former memoir on Pyrosoma (1. ¢. p. 584), 
I have recorded the following observation -— 
“In young specimens, when the ovum is small and the yelk pale, 
this gubernaculum [the duct] frequently appears to be solid; but in 
fully grown specimens, when the ovum [ovisac] has its full size, and 
the yelk is dark and granulous, it presents the appearance of a wide 
tube, especially at its upper part. And here, there was frequently an 
appearance of dark striz and moving granules, prompting the belief 
that spermatozoa had travelled thus far. In one instance the sac of 
the ovum was empty, and the gubernaculum or duct widely dis- 
tended ; the appearance of spermatozoa in the duct was here very 
strong. (Fig. 5.)” 
I entertain no doubt, then, that the specimens described exhibit 
the process of impregnation in Pyrosoma ; that the spermatozoa make 
their way up the duct and come into contact with the surface of the 
yelk. Whether that reciprocal action of the spermatozoon and the 
ovum, which constitutes the essence of fecundation, takes place 
immediately on the occurrence of this contact, I cannot pretend to 
say with certainty, but I doubt it ; for, as will be seen, though very 
remarkable changes take place shortly after impregnation, they are 
not those which in other animals follow upon fecundation. 
It is not a little singular that, in consequence of the immature 
condition of the testis of zooids whose ovisacs are in the stage under 
consideration (Pyrosoma resembling Sa/pa in the much more rapid 
advance towards maturity of the female, than of the male, organ, in 
each zooid), the spermatozoa which effect impregnation must be 
derived from another zooid if not from another ascidiarium. The 
latter alternative is not so improbable as it looks at first sight, if we 
consider that a current constantly sets through the body of each zooid 
from the oral the atrial aperture, and so out at the cloaca. Hence, the 
spermatozoa which are poured by the vas deferens of any given zooid, 
in which the testis has attained its full development, into its atrium, 
must be almost immediately carried into the cloaca; and asa power- 
ful current is setting into the cloaca from every other zooid, it does not 
seem possible that the spermatozoa should make their way into any 
one of these zooids against it. But on the other hand, as the Pyro- 
somata live in great troops, the spermatozoa cast out of the cloaca of 
VOL. II AA 
