2 I 6 PORIFERA 



misled some of the earlier investigators but was established by 

 the researches of SoUas and Zittel. 



Sub-Class 11. Monaxonida.^ 



The Monaxonida inhabit for the most part shallow water, but 

 they also extend through deep water into the abysses, thirteen 

 species having been dredged from depths of over 2000 fathoms 

 by the " Challenger " Expedition alone. In some cases, e.g. 

 Cladorliiza, Ghondrodadia, all the species of a genus may live in 

 deep water, while in others the genus, or in others, again, the 

 species, may have a wide bathymetrical range. Thus Axinella 

 spp. occur in shallow water and in various depths down to 2385 

 fathoms, Axinella erecta ranges from 90 to 1600 fathoms, 

 Stylocordyla stipitata bom 7 to 1600, and so on. The symmetry 

 of the deep-water forms contrasts strikingly with the more 

 irregular shape of their shallow-water allies.^ The shallow-water 

 species are almost always directly attached, some few are stalked ; 

 those from deep water have either a long stalk or some special 

 device to save them from sinking in the soft ooze or mud. 

 Thus the deep-sea genus Trichostemma has the form of a low 

 inverted cone, round the base of which a long marginal fringe of 

 spicules projects, continuing the direction of the somal spicules, 

 and so forming a supporting rim. The same form has been 

 independently evolved in Halicnemia patera, and an approach to 

 it in Xenospongia patelliformis. A similar and more striking 

 case of homoplasy is afforded by the Crinorhiza form, which has 

 been attained in certain species of the deep-sea genera Ghondro- 

 dadia, Axoniderma, and Gladorhiza ; here the sub-globular body 

 is supported by a vertical axis or root, and by a whorl of stout 

 processes radiating outwards and downwards from it, and formed 

 of spicular bundles together with some soft tissue. 



There is recognisable in the order Monaxonida a cleft between 

 one set of genera, typically corticate, and suggesting by their 

 structure a relationship, whether of descent or parentage, with 

 the Tetractinellida, and a second set typically non-corticate : these 

 latter are the Halichondrina, the former are the Spintharophora. 



' Ridley and Dendy, Challenger Monograph, lix. 1887. 

 2 lUd. p. 262 ; cf. also p. 197. 



