_Sponge_ soft, fragile, forming small cushion-shaped masses, grey or green; oscula few in number, often raised on sloping eminences surrounded by radiating furrows below the external membrane; external membrane adhering to the parenchyma.
_Skeleton_ close, almost structureless. "Surface of mature specimens often found covered with parallel skeleton spicules, not yet arranged to form cell-like interspaces" (_Potts_).
_Spicules._ Skeleton-spicules slender, cylindrical, almost straight, sharp or blunt, minutely, uniformly or almost uniformly spined; spines sometimes absent at the tips. No flesh-spicules. Gemmule-spicules with the lower rotula invariably larger than the upper; both rotulæ flat or somewhat sinuous in profile, usually circular but sometimes asymmetrical or subquadrate in outline, varying considerably in size.
_Gemmules_ small, numerous or altogether absent, covered with a granular pneumatic coat of variable thickness; the rotulæ of the gemmule-spicules overlapping and sometimes projecting out of the granular coat.
The measurements of the spicules and gemmules of an Indian specimen and of one from Lehigh Gap, Pennsylvania, are given for comparison:--
Travancore. Pennsylvania. Length of skeleton-spicules 0.189-0.242 mm. 0.16-0.21 mm. (average 0.205 mm.) (average 0.195 mm.) Breadth " " 0.0084-0.0155 mm. 0.0084 mm. Length of birotulate 0.0126 " 0.0099 " Diameter of upper rotula 0.0084 " 0.0084 " " lower " 0.0169 " 0.0168 " " gemmule 0.243-0.348 mm. 0.174-0.435 mm.
The spicules of the Travancore specimen are, therefore, a trifle larger than those of the American one, but the proportions are closely similar.
The difference between the gemmule-spicules of this species and those of such a form as _T. phillottiana_ is merely one of degree and can hardly be regarded as a sufficient justification for placing the two species in different genera. If, as I have proposed, we confine the generic name _Tubella_ to those species in which the gemmule-spicules are really like "little trumpets," the arrangement is a much more natural one, for these species have much in common apart from the gemmule-spicules. _T. pennsylvanica_ does not appear to be very closely related to any other known species except _T. phillottiana_.