156 SPONGES 



unequal in size, and may be straight or tapering, blunt or sharp, smooth 

 or with warts on one surface. 



Should these observations be confirmed, it is evident that we have 

 here two groups of equal systematic importance with the Hexactinellids 

 and Demospongiae, which have not left descendants persisting to our 

 time. In addition, therefore, to the three classes now existing, we should 

 have to add the following : 



CLASS 4. *OCTACTINELLIDA, Hinde. 



With octactinal megascleres. Genus Astraeospongia, Roemer [Sil. 

 Dev.]. 



CLASS 5. *HETERACTI:NELLIDA, Hinde. 



With polyactinal megascleres. Tholiasterella, Hinde [Carb.] ; Aster- 

 actinella, Hinde [Carb.]. 



V. THE DISTRIBUTION OF SPONGES IN SPACE AND TIME. 



The orders and families, and even as a rule the genera, of the 

 Porifera are cosmopolitan in their geographical distribution. Their 

 occurrence in any quarter of the globe is subject only to the re- 

 strictions imposed by the peculiar conditions necessary for their 

 existence in each case, such as, for instance, their bathy metrical 

 distribution, presently to be discussed. Even the freshwater 

 sponges, in spite of the discontinuous nature of their habitat, seem 

 to occur in the lakes and rivers of all countries. In the latter case, 

 the gemmules afford an important means of distribution on account 

 of their resistance to external vicissitudes and the ease with which, 

 in many cases, they can be transported by winds. In marine 

 sponges the larvae are probably often carried great distances by 

 currents, and in some cases gemmules, or other non-sexual repro- 

 ductive bodies, may also play a part in their dispersal. 



Although no group or family of sponges appears to be limited entirely 

 to any particular region, yet many are found more abundantly in certain 

 regions of the globe than elsewhere, and may be said to characterise these 

 areas. Thus in the Hexactinellids a far larger number of species are re- 

 corded from the Pacific than from the Atlantic or Indian Oceans ; this is 

 true both of Lyssacina and Dictyonina ; but while in the case of the 

 former the south temperate zone is the richest and the north temperate zone 

 the poorest in species, the Dictyonina reach their richest development in 

 the tropical zone (Schulze). In the Demospongiae, the Keratosa, a group 

 to which warm and shallow waters seem to be most congenial, are most 

 abundant in southern and antarctic regions (Lendenfeld). The Monax- 

 onida, though a widely spread and very cosmopolitan group, are most 

 abundant in the Indo- Australian region, a fact true especially of the very 

 populous sub-families of the Chalininae and Ectyoninae. Tedania and its 



