REPORT ON THE TETRACTINELLIDA. 
Ixxi 
first stage of transition instead of an oxytylote, not a great difference, and we shall also 
admit that the number of spines may at first have been less definite in number than they 
are now in the fully established trisene. If this is the teaching of ontogeny we might 
expect to find it confirmed by the occasional occurrence of spicules persisting in the 
intermediate stages of development, and these are not necessarily to be sought for in 
closely related families, for if the evolution of spicular forms has been due to the action of 
general causes, we should expect under similar circumstances to find similar forms evolved. 
The existence of the intermediate forms which theory predicates is a matter of notoriety: 
thus in the Desmacidinse, which have probably descended from the same branch as the 
Tetillidse, two closely related species have been described as possessing spicules which 
differ from the strongyloxeas [Rhapliidotheca marshall-halli, Kent,^ and Rhaphidotheca 
affinis, Carter forming the greater part of the skeleton, by the presence of a large tylus 
in place of the usually oxeate distal termination. Again in the Suberites, which are 
probably not very closely related to the Tetractinellida, two quite different species 
{Rrototeleia sollasi^ and Radiella schoenus'^) have been adduced as furnishing similar 
evidence ; in the latter of the two species the tylotoxea, which forms the chief radiating 
spicular fibres of the sponge, occasionally becomes enlarged at its distal end into a large 
tylus, of more or less irregular shape, but sometimes almost spherical and always rough- 
ened by an irregular and minute spination ; in the first-named species a minute cortical 
spicule is present, sometimes of a tylote fqrm, sometimes with the distal tylus produced 
into an unascertained number of recurved spines. The resemblance of this spicule to an 
anatrisene is obvious, though the number of the cladi, for such the recurved spines may 
be fairly termed, does not appear to have attained that constancy which usually 
distinguishes the trisene ; Dendy and Eidley state that though they could not quite 
convince themselves yet they believe the number to vary from three to four. In Acarnus 
innominatus, as Dendy and Eidley point out, a tyloclad exists with a cladome of four 
cladi, and in Acarnus ternatus a similar spicule with three. These instances, however, 
only suggest the independent origin of cladose spicules in different groups of sponges, and 
do not furnish us with persistent intermediate stages, as do the distally tylote rhabdi of 
Esperia marshall-halli and Radiella schoenus ; to guard against misconception I may as 
well add that I do not for a moment suppose that the spicules last alluded to stand in- 
any close genetic relation to the true trisene, as it exists in the Tetractinellida, they also 
are independently evolved forms, but persist in a stage through which we may assume 
the trisene to have passed. In the succeeding chapter on the origin of spicular forms I 
shall attempt to show that a rhabdal origin of the trisene is not inconsistent with a 
general theory of spicules. 
'■ Ann. and Mag. Nat. Hist., ser. 5, vol. i. pp. 4, 6, pi. xv. 
Journ. Roy. Micr. Soc., vol. ii. p. 497, pi. xvii. figs. 1-34. 
® Dendy and Eidley, Ann. and Mag. Nat. Hist., ser. 5, vol. xviii. p. 153, pi. v. 
* Sollas, op. cit., ser. 5, vol. ix. p. 163. 
