36 NATURAL SCIENCE. July. 



calcareous sponges on the one hand, and in the remaining sponges 

 on the other. Nature, having presented to her the problem — how 

 to obtain by means of flagellated cells a maximum current with 

 minimum expenditure of energy — has solved it in two different 

 groups of organisms in essentially the same way ; and the method 

 of solution is one which, on physical principles, appears to be 

 inevitable. 



The number of species of Calcarea, all described from " Chal- 

 lenger" material, is thirty, of which twenty-three were new ; they are 

 grouped in twelve genera, of which four were new. (PI. vi., Fig. 2.) 



Polejaeff' s work on the Keratosa contains a large amount of 

 exact information on the anatomy and histology of these sponges, but 

 the material at his command was in this case too limited to afford 

 sufficient data for a satisfactory system of classification : indeed, to 

 arrive at this, a study of the Keratosa alone is inadequate, they must 

 be taken in connection with the Monaxonida, from which the 

 majority of them have almost certainly been derived. 



Thirty-four species of Keratosa were obtained by the " Chal- 

 lenger," of which twenty-one proved to be new ; they are distributed 

 among twelve genera, none new. 



It is to the Hexactinellida that the greatest interest naturally 

 attaches. The exquisite beauty of such forms as Venus' Flower- 

 Basket, the marvellous symmetry of complicated spicules which 

 many well-known members of the group afford to the microscope, and 

 the long acquaintance which the palaeontologist has had with these 

 sponges in the fossil state previous to their revelation as living forms 

 by the dredge, have rendered them remarkable in popular esteem. 

 Till, however, Schulze undertook their investigation, they were still 

 objects of mystery, for nothing was known of the structure of the 

 sponge itself. It was a great opportunity when the " Challenger " 

 collection was placed in Schulze's hands, and splendidly he employed 

 it. By means of the microtome all the well-preserved specimens were 

 laid open to precise observation, and on the exact knowledge of 

 structure thus obtained was based the foundation of a natural system of 

 classification. The arrangement of the soft parts and the character 

 of the chamber-system were found to be singularly uniform ; in the 

 latter a syconate character prevailed and indicated the position of 

 the Hexactinellida as the lowest of the siliceous sponges, a fact in 

 harmony with their very early appearance in the stratified series of 

 the earth's crust. The uniformity of the chamber-system and the 

 persistence of a sexradiate form in the spicules, sharply mark off the 

 Hexactinellida from all other siliceous sponges ; they form a truly 

 natural order — an enviable position amongst sponges, to which the 

 Calcarea alone among the rest have like claim. 



If the canal-system is uniform and thus of no use for classifica- 

 tory purposes within the group, the spicules are just the reverse, and 

 by their extreme diversity offer characters of the highest taxonomic 



