Ix THE VOYAGE OF H.M.S. CHALLENGER. 



III. Discussion of the Different Subdivisions. 



The Monaxonida thus comprise two fairly natural suborders, the Halichoudrina and 

 the Clavuhna. But whether these two suborders are sufficiently closely related to one 

 another and sufficiently distinct from other suborders to admit of their Ijeing united in 

 one natural order, the Monaxonida, as opposed to the Tetractinellida, is very doubtful. 

 In the classification of Dr. Vosmaer the order Monaxonida finds no place, and we are 

 inclined to agree with this authority, who accepts the suborders Halichondrina and 

 Clavulina as themselves natural, but refuses to allow a special order for the reception of 

 these two groups to the exclusion of others. There is much evidence in favour of this 

 view. The mere possession by the Tetractinellida of tetraxonid spicules is no safe 

 guide. Imperfectly developed " grapnel spicules," which in a more highly developed 

 condition are so characteristic of Tetractinellid sponges, are now known to occur in 

 the Suberitidas {Fi'oteleia) , and polyaxonid megasclera are occasionally met with in 

 Desmacidonidae and Axinellidse (Acarnus and Thrinacophora). Again, the test whether 

 a sponge is corticate or not breaks down utterly in this case, for the Clavulina, like some 

 of the Tetractinellida, are nearly all corticate and have mostly a radially disposed 

 skeleton. We even find a cortex, associated with a radially disposed skeleton, in one 

 genus of Desmacidonidse (Phelloderma). It is quite certain that there exists no sharp 

 line of division between the Monaxonida and the Tetractinellida, for the Suberitidse, 

 Spirastrellidge and Tethyadae supply us with abundant connecting links. 



That either the Tetractinellida have been derived from the Monaxonida, or vice 

 versd, is now a generally accepted fact, but which is the parent group is a matter of 

 much controversy, and there is much to be said on both sides. We have already^ 

 advanced strong reasons for supposing the Tetractinellida to be derived from the 

 Monaxonida, whilst Vosmaer upholds the contrary hypothesis. The time has as yet 

 scarcely arrived when a satisfactory discussion of the question is practicable. We must 

 wait for more evidence, and the evidence chiefly to be desired is of an embryological 

 character. In any case it appears to us that the Clavulina and Tetractinellida are at any 

 rate as intimately connected with one another as are the Clavulina and Halichondrina. 



That the Keratosa are most closely connected with the Halichondrina is also now a 

 generally accepted fact, which finds its expression in recent classifications. They are 

 probably Halichondrine sponges, which, living in warm seas, have developed a large 

 amount of spongin, and suff"ered a correspondingly great reduction in the proportion of 

 spicules present in the skeleton. We can trace this development of spongin through 

 all intermediate stages ; through the Eenierinse to the Chalininae, and thence to aspiculous 

 forms. But this is not the only path by which the same results may have been arrived 

 at. The Challenger dredgings teach us that a horny skeleton may be developed in the 



' Ann. and Mag. Nat. Hist., ser. 5, vol. sviii. p. 152, et seq. 



