2 20 PORIFERA 



Sub-Class III. Ceratosa. 



The Ceratosa are an assemblage of ultimate twigs shorn from 

 the branches of the Monaxonid tree. They are therefore related 

 forms, but many of them are more closely connected with their 

 Monaxonid relatives than with theu- associates in their own sub- 

 class. 



The genera Aulena and Phoriospongia, placed by v. Lendenfeld 

 among Ceratosa, by Minchin among Monaxonida, show each in 

 its own way how close is the link between these two sub-classes. 



Aulena possesses in its deeper parts a skeleton of areniferous 

 spongin fibres, in fact a typical Ceratose skeleton ; but this is 

 continuous with a skeleton in the more superficial parts, which is 

 composed of spongin fibres echinated by spicules proper to the 

 sponge, and precisely comparable to the ectyonine fibres of some 

 Monaxonida. 



Phoriospongia, as far as its main skeleton is concerned, is a 

 typical Ceratose sponge, with fibres of the areniferous type, but 

 it possesses sigmata free in the flesh. 



The sub-class is confined to shallow water, no horny sponge 

 having been dredged from depths greater than 410 fathoms.-' The 

 greatest number occur at depths between 10 and 26 fathoms. 



In the majority of the Ceratosa the skeletal fibres are homo- 

 geneous, formed of concentric lamellae of spongin, deposited by 

 a sheath of spongoblasts around a filiform axis. In others, 

 however, the axis attains a considerable diameter, so as to 

 form a kind of pith to the fibre, which is then distinguished 

 as heterogeneous. In one or two cases some of the spongo- 

 blasts of a heterogeneous fibre are included in the fibre between 

 the spongin lamellae. lanthella is the best -known example 

 in which this occurs. 



Ceratosa are divided into Dictyoceratina and Dendroceratina, 

 distinguished, as their names express, by the nature of the 

 skeleton — net-like, with many anastomoses, in the one ; tree-like, 

 without anastomoses between its branches, in the other. 



The Dictyoceratina comprise by far the larger number of 

 Ceratosa. They fall into two main families, the Spongidae and 

 Spongelidae, both represented in British waters. The Spongidae 



' R. von Lendenfeld, Monograph of Horny Sponges, 1889, p. 831. 



