498 THE VOYAGE O'F H.M.S. CHALLENGER. 



I may therefore regard it as indubitable that in many cases the regular and typical 

 tetraets have, by gradual reduction of individual rays, given rise to diacts and even 

 monacts. I do not mean to say that all diacts and monacts have originated in tetraets. 

 On the contrarj^, previous investigators of the Hexactinellida have shown, what I think 

 I have also clearly demonstrated, that in this group at least the very abundant and 

 richly developed diacts and monacts have arisen, not from the regular tetraets of 

 the Tetraxonia (the so-called chevaux de frise), but from the regular hexacts of the 

 Triaxonia. But while there are numberless extant transitions from the Tetraxonia with 

 typical tetraets to the strict Monaxonia with only straight diacts or monacts, there are 

 among living, and, so far as I know, among fossil sponges no transitions from the 

 Triaxonia to strict Monaxonia, so that we have no reason for the supposition that the 

 latter have been evolved from the former. The case is somewhat different in regard to 

 some sponges without a skeleton, lately discovered in Australia by von Lendenfeld (such 

 as Bajalus') in which the structure agrees so closely with the soft parts of several 

 Hexactinellids that one is inclined to suppose their derivation from the latter by a total 

 loss of siliceous spicules. 



Under these circumstances, the supposition is legitimate, that all the Monaxonia, and 

 the Keratosa which have probably developed from them, have originated from the stem 

 of the Tetraxonia. And since the spicules of the Lithistidae, as 0. Schmidt, Zittel, and 

 others have conclusively shown, are derivable from the regular tetract type, we may thus 

 regard the Tetraxonia with simple regular tetraets as the starting point for all the flinty 

 and horny sponges except the Hexactinellida. 



For the possibility that the Hexactinellida also stand in genetic relationship with the 

 Tetraxonia, I find no basis of facts. In 1870 0. Schmidt^ expressed the same opinion in 

 the following words : — " Between the type of spicule in which the rays are determined 

 by the three-sided pyramid and the triaxonial type, there are, so far as the forms go, no 

 relations. The sponges in which these two types occur appear to be distinct and separate 

 independent twigs, in regard to which one must distinguish clearly between the general 

 homologies and the adaptive analogies." 



We thus reach this conclusion, that the sponges may be grouped along three main 

 stems, which may indeed be regarded as springing from a common root — a very simple 

 primitive sponge without a skeleton — but which remain separate from this root onwards, 

 without exhibiting any connecting links. 



This may be expressed in the form of a genealogical tree (see fig. 9). 



Now if we may regard it as probable that each of these three main stems, which 



represent the divisions of the great crowd of sponges, namely (1) Calcarea, (2) Tetraxonia 



« with Monaxonia and Keratosa, and (3) Triaxonia, possessed to start with, either exclusively 



' Proc. Linn. Soc. N.S. IF., vol. s. p. 5. 



^ Grundziige einer Spotigienfauna des atlantischen Gebietes, 1870, p. 5. 



