SPONGES 163 



From a corticate Choristid with a radiate skeleton the step to 

 Tethya and allied forms is not great. The triaenes are replaced by 

 monaxons, orientated in a similar manner to the rhabdomes of the 

 triaenes, and doubtless corresponding to them. By reduction of the 

 tetraxons the sponge has now become a Monaxonid, the starting-point 

 of a new evolutionary series. While many Monaxonida, especially of 

 the order Hadromerina, differ little from typical Choristida except 

 in the absence of tetraxon spicules, others by reduction or dis- 

 appearance of the cortex, absence of the characteristic asters 

 amongst the microscleres, shortening and diminution of the monaxon 

 megascleres, and loss of their radiate arrangement, acquire a type 

 of structure which in the end terms of the series is of a very 

 distinct character. While the Hadromerina retain, as an order, 

 many marks of Tetractinellid affinities, all traces of the latter 

 become obliterated more or less completely in the Halichondrina, 

 with their reticulate type of skeleton, held together very often by 

 an element which was absent or very inconspicuous in the Hadro- 

 merina and Tetraxonida, namely, spongin. 



The Halichondrina in their turn are the starting-point "of an 

 evolution in yet another direction, in which the spicules become 

 gradually lost and replaced by spongin, which ultimately comes to 

 make up the whole skeleton. The transitions between- the 

 Halichondrina and Keratosa are numerous and gradual, and, as 

 already pointed out above (p. 139), the evolution of a so-called horny 

 sponge has probably taken place in more than one family of 

 Halichondrina, and perhaps even more than once within the limits 

 of the same family. So inseparable are the Keratosa, or rather the 

 Dictyoceratina, from the Halichondrina in a natural classification 

 that it has been proposed by Vosmaer and others to unite them in 

 one group Cornacuspongiae, and so separate them from the other 

 Demospongiae which, in their turn, are to be united in one class 

 Spiculispongiae, comprising the Tetraxonida and Hadromerina. This 

 arrangement, however, is open to just the same objections as that 

 which it is intended to replace, namely, that it introduces sharp 

 cleavages where none naturally exist, and for practical purposes it 

 is less convenient than the frankly artificial classification into 

 Tetraxonida, Monaxonida, and Keratosa. 



It is seen that from the most primitive Tetraxonida to the typical 

 horny sponges we find an uninterrupted series of gradations, and though 

 it might be possible to link the various forms together along lines different 

 from those which have been traced above, it is not possible to introduce sharp 

 distinctions between them. On the other hand, in the Keratosa themselves 

 we come perhaps for the first time to a discontinuity in the chain of forms. 

 The two orders of the Keratosa seem to have little in common except the 

 material of the skeleton, and Leridenfeld has sought to bring the Dendro- 

 ceratina near to the Hexactinellids, on the supposition that they represent 



