SPONGES 157 



allies, on the other hand, are more characteristic of the Patagonian region 

 (Ridley and Dendy) ; while the higher systematic groups of sponges have 

 the widest possible distribution, the range of individual species is often 

 very restricted, though certain forms may be of widespread occurrence. 

 Instances of the latter are seen in some generalised forms, such as 

 Halichondria panicea, Sitberites carnosus, and many others amongst 

 Monaxonida. The species inhabiting deep water occur, as a rule, over 

 wider areas than those restricted to the shore-line. 



The classes of Porifera are better characterised by their bathy- 

 metrical distribution than by their geographical habitat. Speaking 

 generally, it may be said that the Calcarea and Monaxonida are 

 shore forms, inhabiting the highest littoral zone, and flourishing 

 between tide marks ; the Choristida, amongst Tetractinellids, and 

 the Keratosa are most abundant just below tide marks, down to 

 about 50 fathoms ; the Lithistida characterise a slightly deeper 

 belt, from 100 to 150 fathoms; while the Hexactinellids are 

 typical inhabitants of deep water, the Dictyonina occurring in 

 moderate depths, near the coast, and the Lyssacina in the abyssal 

 regions far from the coast. In every case, however, the limits of 

 these generalisations are overstepped by particular species. Thus 

 Thenea muricata (Choristida) has been recorded from 1913 fathoms 

 (Wright), and many Monaxonida have spread down to great 

 depths, as, for example, Cladorhiza longipinna (R. and D.) from 

 3000 fathoms. It has already been pointed out as an interesting 

 fact that the influence of an abyssal habitat upon these character- 

 istic littoral forms, is to cause them to exchange their irregular 

 body form for a symmetrical mode of growth, which is clearly 

 secondary and newly acquired. 



The geological record of the Porifera is largely dependent, as 

 might have been expected, upon the nature of the skeleton. In 

 each group we find those forms especially represented by fossils in 

 which a rigid and coherent framework has been evolved ; for 

 instance, amongst Calcarea, the Pharetronidae ; amongst Hex- 

 actinellids, those with a dictyonal framework ; and amongst 

 Demospongiae, the Lithistida. Since in each case -these are the 

 least primitive examples of the groups to which they belong, 

 Palaeontology affords us but little help in unravelling the phylo- 

 genetic connections of the groups of Porifera. We know nothing 

 of the past history or distribution of the more primitive groups, 

 such as the Ascons or the Carnosa. The most ancient sponge 

 known, however, namely, Protosjwngia from the Cambrian, is 

 characterised by triaxon spicules of the most primitive and un- 

 modified type, and the early Palaeozoic forms classed by Hinde as 

 Octactinellida and Heteractinellida perhaps represent oftshoots from 

 a very early and primitive stock, which have not left descendants 

 persisting to the present day. On the other hand, a remarkable 



