Ixii THE VOYAGE OF H.M.S. CHALLENGEE. 



Monaxonida, altliougli we now feel that the group thereby designated has no place in a 

 natural classification. 



The Halichondrina are, taken as a whole, strongly contrasted with the Clavulina. 

 Members of the former group very rarely indeed possess a true, fibrous cortex, while 

 those of the latter are rarely without one. In the former group again the skeleton is 

 usually more or less reticulate, in the latter group it is usually radiately disposed, with no 

 secondary, crossing fibres. Thirdly, the Halichondrina have always, or nearly always, a 

 greater or less amount of spongin in the skeleton, which is absent in the Clavulina. All 

 these are important distinguishing characters which, taken in connection with the difier- 

 ences in spiculation, are quite sufficient to separate the two suborders. 



Yet here, as in other cases, the line of division is not absolute, for we have, in the 

 Axinellidse, transitional forms. Hence we have placed the Axinellidje next to the 

 Suberitidse and at the end of the Halichondrina. In the Axinellidse the skeleton shows a 

 strong tendency to radiate arrangement, and the spicules are generally monactinal, as in 

 the Clavulina ; moreover, the microsclera, when such are present, appear to be sometimes of 

 a distinctly Clavulinid type ; viz., stellate. As yet we know no Axinellid sponge with a 

 true fibrous cortex, but, on the other hand, there are species referred to the genus 

 Suherites, in which the presence of a fibrous cortex has not yet been demonstrated, 

 although they possess the characteristic tylostylote spicules. The two genera Suherites 

 and Axinella appear to be nearly related. This connection may be a less intimate one 

 than we at present think, but this can only be decided when our now very imperfect 

 knowledge of these sponges has been greatly augmented. This is the most apparent point 

 of contact between the Halichondrina and the Clavulina, unless Schmidt's description of 

 his Sceptrella regcdis^ should prove to be correct, and then we should be forced to imagine 

 a connection between the Spirastrellidas and the Esperellinse. Sceptrella regalis is 

 described as j^ossessing discastra (spicules characteristic of the genus Latrunculia) 

 and, at the same time, chelae of a very peculiar type. It is placed by its founder 

 amongst the Desmacidines. There is, presumably, only a single specimen of this 

 sponge. It is described as " eine graue, diinne Kruste," and no account is given of 

 the megasclera. Having regard to the extremely unlikely combination of discastra 

 with chelae, and to the fact that the sponge is known only by a thin crust, and 

 bearing in mind also the frequent occurrence in sponges of spicules belonging to other 

 species, we are strongly inclined to believe that Schmidt had before him a youn» 

 Spirastrellid sponge growing over the remains of some species belonging to the Esperel- 

 linae, and that the discastra belong to one sponge and the chelae to the other.^ 



^ Spong. Atlant. Gebiet., p. 58 ; cf. also p. 234 of the present work. 



2 This view of the case is very strongly supported by our examination of a preparation of the spicules of Sceptrella 

 regalis (laljelled in Schmidt's handwriting) in the British Museum. We find that the discastra and megasclera are 

 typically those of a Latrunculia. The latter are smooth, sharp-pointed styli, agreeing with those of other species of 

 Latrunculia even down to the characteristic slight crookedness of the shaft {cf. PI. XLV. figs. 8, 9, 10), and measuring 



