CALCAREA. 89 



fact, if we had to set forth in a pictorial manner the true affinities of the 

 families of this sub-class, we should be obliged to use a network or, better still, 

 a sponge work, rather than a tree or a line, because so many of the families 

 present affinities of apparently equal importance in more than one direction. 

 And this applies with equal force to genera and species. If this vittf of a 

 reticular arrangement rather than that of a tree arrangement were generally 

 held, we feel assured that a good many of the difficulties of sponge classification 

 would be disposed of. It is the genealogical tree idea which makes the difficulty. 



Fossil sponges are found in various formations, e.g., in the chalk, 

 and these remains appear to differ considerably from those now 

 living. The Hexadinellida, however, agree so fully with the ancient 

 forms that they might be direct descendants of them. Finally many 

 of the principal groups extend back into the palaeozoic age, in which 

 the Lithistida and Hexactinellida especially are found in the most 

 ancient Silurian strata. 



Class I. CALCAREA.* 



Sponges with a calcareous skeleton and large choanocytes. 



Order 1. CALCAREA. 

 With the characters of the class. 



Sub-order 1. HOMOC(ELA. 



Calcarea without flagellated chambers ; the internal surface entirely lined by 

 collared cells. 



Fam. 1. Asconidae. Gastric cavity a simple sac. Leucosolenia Bow. 



Polejaeff and Dendy both unite 

 all Homoccela in one genus, for 

 " the spicules of the Calcarea, 

 being very variable in every direc- 

 tion, could not serve as a basis 

 for the distinction of genera." 

 Haeckel's genera therefore go. 

 Dendy distinguishes three sections 

 of the genus: (1) Olynthus types 

 which do not form colonies, or, if 

 they do form colonies, in which 

 the individuality of the members 

 of the colony (Ascon- persons) is 

 always recognisable (Fig. 77); (2) FIG. 77.-Section through an Ascon-colony, 



colonies the members of which diagrammatic (after Haeckel). 



anastomose and form a network ; 



(3) colonies consisting of a central Ascon tube from which other tubes are 

 radially budded off. 



* A. Dendy, "A Monograph of the Victorian Sponges," part 1. Transactions 

 of the Royal Society of Victoria, vol. 3, part 1. A. Dendy, "Observations on 

 th 



e Structure and Classification of the Calcarea Heteroccela." Q. J. M. ., vol. 

 , 

 vol. 



35, 1893, p. 159. N. Polejaeff, "Report on the Calcarea." Challenger Jieports, 

 l. 8, 1883. 



