CALCAREOUS SPONGES. 715 



the group. It would obviously be impossible, in any natural 

 systematic arrangement, to associate together all the species 

 which possess microxea, for these species differ amongst them- 

 selves in nearly every other respect. It is noteworthy that in 

 some cases these spicules occur in very small numbers, so that 

 they might easily be overlooked, while in others they are ex- 

 tremely numerous ; and it seems by no means impossible that 

 some individuals of a species may possess them while others do 

 not. Nevertheless, as a matter of convenience, we have decided 

 to make use of the presence or absence of these spicules for the 

 purpose of distinguishing sections of genera. 



In this connection, however, it must be observed that we do not 

 include, in our conception of the term microxea, those long, hair- 

 like spicules frequently found surrounding the osculum, or some- 

 times echinating the surface. These we believe to be merely 

 slightly modified or imperfectly developed large oxea, and we 

 include them under that head, under the term ' trichoxea.' 



One is tempted to explain the sporadic distribution of oxea by 

 speculations which, in the pi-esent state of our knowledge, are 

 perhaps unjustifiable ; but we may perhaps venture to suggest 

 that the presence of oxea constituted a characteristic feature of 

 some remote ancestor, and that the faculty of producing them has 

 never been entirely lost, but requires special genetic conditions 

 of which we know nothing before it can become active in any 

 particular species. It is quite possible that our sections are 

 somewhat artificial, but a grouping of the species by easily 

 recognisable characters, especially in the larger genera, can hardly 

 fail to be of use to the systematist. 



Further discussion of the principles of classification may con- 

 veniently be left until we come to deal with the various sub- 

 divisions of the group. 



SYSTEMATIC ARRANGEMENT OF THE CALCAREA. 



Class and Order CALCAREA. 



Diagnosis. Sponges in which the spicules are composed of car- 

 bonate of lime (calcite), and consist of either triradiate or 

 quadriradiate systems, or are oxea (monaxons). 



For many years past it has been the almost universal practice 

 amongst spongologists to divide the class Oalcarea into two 

 sharply contrasted oi-ders, viz. Homoc(ela, in which the whole 

 of the gastral cavity is lined by collared cells, and Heteroccela, 

 in which the collared cells are confined to special flagellate 

 chambers, a practice which was first initiated by Polejaeff in 



