818 
DE. J. S. BOWEEBANK ON THE ANAT03^IT 
perfect accordance with them. In Tethea cranium the same mode of reproduction by 
gemmules obtains, but the form of the organ is different, and there are other peculi- 
arities in its growth and development that are extremely interesting. 
The form of the gemmules is regularly lenticular; and there are two distinct sorts of 
them, Avhich are always grouped together. The first is rather the smaller of the two, 
and has a nucleus of slender curved fusiformi-acerate spicula only. The bases of the 
spicula cross each other at the centre of the gemmule, and the apices radiate in all 
directions towards the external surface, but do not, in the fully developed state of the 
gemmule, project beyond it. The second sort of gemmule is furnished with three 
distinct forms of spiculum. The first are like those of the gemmule described above, 
slender fusiformi-acerate ; the second are attenuato-porrecto-ternate, the radii being 
given off from the apex at about an angle of 45 degrees ; and the third form is 
attenuato-bihamate or unihamate, and the hooked apices of this form are projected 
further than either of the other two forms, but do not pass beyond the inner surface of 
the tough dermal envelope of the gemmule when in the adult state. I have examined 
a great number of these gemmules, and could never find in the form first described any 
indication of either ternate or hamate spicula, and I am therefore satisfied that they are 
separate descriptions of gemmule, and that the first form is not a transition state from 
the young and undeveloped to the fully developed one. In like manner I have closely 
observed the second form, and have always found it uniform in character, and furnished 
with the whole three forms of spicula that characterize it. It is highly probable that 
this marked difference in structure is sexual, and, from the more highly developed con- 
dition of the second or large form, that it is the female or prolific gemmule ; but on this 
point we must at present be satisfied with conjecture only, as although I have searched 
diligently for spermatozoa in both forms of gemmule and in the surrounding sarcode, I 
have not been able to detect anything resembling them. But that such bodies do 
occur in some species of Tethea appears to be the case. Professor Huxley ha'\ing 
described and figured bodies which he believed to be spermatozoa in a paper published 
in the ‘Annals and Mag. Nat. Hist.’ Second Series, vol. vii. p. 370, plate 14, as occurring 
in a species of Tethea found in one of the small bays in Sydney Harbour, Australia. 
The group of gemmules represented by fig. 1, Plate XXXV., consists of (a) one of the 
larger and supposed prolific gemmules, and three (h, b, b) of the presumed male gemmules 
in situ, Xl08. Wherever the former occurs the latter appear always to accompany 
them in the proportion of about two or three to one. They are not seated like the 
ovaria of Geodia at the surface of the sponge, but are always found on the interstitial 
membranes at a considerable depth within the sponge. The immersion of the specimen 
in Canada balsam has rendered the marginal lines of the gemmules undistinguishable 
from the surrounding sarcode, but their natural boundaries would be just beyond the 
extreme points of the spicula. 
Fig. 2, Plate XXXV. represents one of the larger gemmules in its natural condition 
and separated from the sponge, by direct light and a linear power of 50. Figs. 39, 40, 
