Ixx 
THE VOYAGE OF H.M.S. CHALLENGER. 
In the Stellettidse the trisenes, as in all trisenose sponges, always both in the adult and 
young sponge, give evidence of an early oxytylote stage {vide p. 144, PI. XIII. figs. 
16, 20). 
The morphological argument appears scarcely less strong than the ontological ; thus 
the form of the rhahdome of the trisene is in many cases, especially within the Tetillidse, 
suggestive of a rhabdal origin. It is frequently fusiform, and so closely resembles the 
associated oxeas, that with the cladome removed it cannot be distinguished from them. 
The resemblance is enhanced by the anisoactinate character of the oxeas, the centrifugal 
actine of which is usually far shorter and therefore apparently stouter than the centripetal. 
The fusiform anisoactinate oxea has every appearance of homology with the anisoactinate 
fusiform rhabdome of the trisene ; the morphological centre of the oxea lies between the 
two actines, and if the rhabdome of the trisene is homologous with it, its morphological 
centre likewise lies somewhere in the middle of its thickest part and not at the cladal 
origin. 
Another point in favour of the rhabdal origin of the trisene may possibly be found in 
the extension of the rhabdome beyond the cladal origin {vide p. 13, PI. V. figs. 8, 9). The 
existence of the centrotrisene and the amphitrisene also suggest a secondary rather than a 
primary origin for the cladome. 
Finally, the position of the nucleus of the scleroblast is a strong point in favour of 
homologising the rhabdome of the trisene with a rhabdus; thus it is situated in the case of 
the Tetillid trisene upon the rhabdome at or about the point where its diameter is greatest, 
and it occupies a similar position in the associated oxeas, so that if we assume that the 
nucleus corresponds in position with the actinal centre, the homology of the rhabdome 
with the rhabdus would be proved. In the trisenes of other Tetractinellida, not Tetilhdse, 
the scleroblastic nucleus never occurs in the region of the cladal origin but always from 
about one-third to one-fifth the length of the rhabdome distant from it. The only fact- 
which I have come across at all opposed to the view here advocated is furnished by the 
very young dichotrisenes of Thenea and Stelletta, these sometimes present a rhabdome 
which is shorter than the cladi, and since in the adult sponge the rhabdome is several times 
longer than the cladi, it follows that the scleroblastic nucleus may be shifted away from 
the cladal origin with the growth -of the rhabdome, and thus that its position in the adult 
spicule is a secondarily acquired one. I have never seen the scleroblast of these young 
dichotrisenes, and so can say nothing as to the position of its nucleus, but in the Tetillidse 
it is situated in the young trisenes as far from the cladal origin as in the adult. 
Again certain general considerations may be adduced in favour of the rhabdal origin 
of the trisene ; thus if we trust to the ontogenetic evidence furnished by the Tetillidse 
and the Stellettidse, we shall be led to suppose that the rhabdus first became modified 
into an oxytylote and subsequently spines originated from the tylus, and if instead of 
these families we consider the Theneidse, we shall have to assume an oxystrongyle as the 
