240 PORIFERA 



richer, for the station curve {a') rises abruptly from I. to II., 

 while the species curve {a) in the same region is almost 

 horizontal. 



The Hexactinellid curve (c) culminates on III., showing that 

 the group is characteristically .deep water. That for Tetracti- 

 nellida (d) reaches its greatest height on II., i.e. between 51 

 and 200 fathoms. Even here, in their characteristic depths, 

 the Tetractinellida fall below the Hexactinellida, and far below 

 the Monaxonida in numbers. Again, the Monaxonida are 

 commoner than Hexactinellida in deep water of 201 to 1000 

 fathoms, and it is not till depths of 1000, fathoms are passed 

 that Hexactinellida prevail, finally preponderating over the 

 Monaxonida in the ratio of 2 : 1. 



The Calcarea and Ceratosa are strictly shallow-water forms. 

 It is a fact well worth consideration that the stations at which 

 sponges have been found are situated, quite irrespective of depth, 

 more or less in the neighbourhood of land. In the case of Cal- 

 carea and Ceratosa this is to be expected, seeing that shallow 

 water is commonest near land, but it is surprising that it should 

 be true also of the Hexactinellida and of the deep-water species 

 of Tetractinellida and of Monaxonida. 



While the family groups are cosmopolitan, this is not true of 

 genera and species. The distribution of genera and species 

 makes it possible to define certain geographical provinces for 

 sponges as for other animals. That this is so, is due to the 

 existence of ocean tracts bare of islands ; for ocean currents can 

 act as distributing agents with success only if they flow along a 

 coast or across an ocean studded with islands. It is, of course, 

 the larval forms which will be transported ; whether they will 

 ever develop to the adult condition depends on whether the 

 current carrying them passes over a bottom suitable to their 

 species before metamorphosis occurs and the young sponge sinks. 

 If such a bottom is passed over, and if the depth is one which 

 can be supported by the particular species in question, then a 

 new station may thus be established for that species. 



The distance over which a larva may be carried depends on 

 the speed of the current by which it is borne, and on the length 

 of time occupied by its metamorphosis. Certain of the ocean 

 currents accomplish 500 miles in six days ; this gives some idea 

 of the distance which may intervene between the birthplace and 



