MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
577 
at its extremity an oval cellular mass, Plate XVL fig. 8. This hangs suspended by 
its pedicle in the cavity of the sinus, and is freely bathed by the blood. In one spe- 
cimen the length of the pedicle was a-i^th of an inch, the long diameter of the oval 
body about of an inch. 
In still younger forms of the Salpa B., and indeed as soon as the separate organs 
are distinguishable, the outer tunic bulges slightly in the middle line behind the out- 
line of the posterior aperture and beneath the nucleus, Plate XVI. figs. 1, 2 ; this pro- 
tuberance is caused by the presence of a spherical body (q) about TWof^^ of an inch in 
diameter, containing a clear vesicle of an inch in diameter, which again fre- 
quently contained a round opake spot or nucleus about - go^oo th of an inch in dia- 
meter ; the latter sometimes appeared as a thick-walled vesicle. This is plainly an 
ovum ; a narrow pedicle {q') is attached to its upper extremity and runs upwards, 
curving slightly forwards to the same point as in the preceding forms. 
It would appear then that the Salpa B developes a single ovum, which is at first 
placed in the median line in the ventral sinus ; that partly by the increase in size of 
the body, and partly in consequence of a shortening of its pedicle which acts as a 
gubernaculum, it becomes drawn from this position upwards and to the left side ; 
and that in the meanwhile, probably in consequence of fecundation, it becomes 
altered in structure, and precisely similar to and identical with the cellular mass 
which has been seen to form the rudiment of the young Salpa A, Plate XVI. fig. 7- 
In this case the Salpa A would be a true embryo developed by a process of sexual 
generation. 
34. Sexual generation however presup|)oses a male fecundating organ, and this is 
found in Salpa B as a ramified body, hitherto generally called a liver (/>), Plate XV. 
figs. 6 and 7, closely surrounding the intestinal canal with a network, solid in the 
younger form, but in the older tubular, with very thin walls, and containing a vast 
number of pale-greenish circular cells, from -g-^o^th to TTVo^h of an inch in diameter ; 
and besides these detached spermatozoa, with very thin tails and long narrow heads, 
about - 16 ^ 0 o th of an inch in length. The testis had no visible excretory organ, but 
such might well escape notice. 
Nothing at all resembling this body is found in the form A ; its contents suffi- 
ciently demonstrate its real nature, and its existence on the other hand is strong 
confirmatory evidence, if any be needed, that the pediculate body described above is 
a true ovum. 
One curious circumstance needs to be remarked ; the testis does not develope pari 
passu with the ovum and attain its full development at the same time, as might be 
imagined. The testis is always behind the ovum in its progress, and does not, 
indeed, seem to have attained its full development until the latter has become freed 
from the parent. 
Without carefully tracing the form B through all its stages, it might readily be 
supposed to be always male ; in fact, fully-grown specimens, while they always possess 
