1 62 SPONGES 



The consequence was the evolution of sponges along two lines, characterised 

 each by the material composing the supporting framework of the body. 



Whatever may have been the first origin of sponge skeletons, that of 

 the siliceous sponges shows two distinct lines of evolution. In Hex- 

 actinellids the starting-point was a form of simple structure with triaxon 

 spicules. The Demospongiae may be referred back, similarly, to a 

 primitive Rhagon form with tetraxon spicules. In both cases the form of 

 spicule may be explained as an adaptation to the canal system and 

 architecture of the primitive sponge (Schulze). The primitive hexactines 

 of the triaxon type are of the form best suited to the elongate, thimble- 

 shaped chambers, and the loose trabecular structure of a simple Hexac- 

 tinellid, while the tetraxon spicule fits the closely packed rounded chambers 

 and the denser texture of the body wall of a Flakina-likc ancestor. The 

 question arises whether the two types of body structure were evolved 

 before or after the sponge had acquired a siliceous skeleton of some kind. 

 It is possible that a remote, ancestral Myxosponge, with flagellated chambers 

 opening into a gastral cavity, secreted siliceous concretions and sclerites, 

 which became spicules of a more or less irregular polyaxon form, and that, 

 as the canal system of this sponge developed in one or the other direction, 

 the polyaxon spicules became adapted to its structure and gradually settled 

 down, so to speak, into the two types represented by the triaxon and 

 tetraxon megascleres respectively. If this be so, we might expect to find 

 other lines of sponge descent, in which the primitive polyaxon sclerite had 

 given rise to other types of spicule, and such forms are perhaps repre- 

 sented by the Palaeozoic sponges placed by Hincle in the two groups named 

 by him Octactinellida and Heteractinellida. On this view we may 

 recognise a class Silicea, of equal phylogene'tic value with the Calcarea, 

 and divisible into two branches, the Triaxonia or Hexactinellids and the 

 Tetraxonia or Demospongiae. It remains to consider briefly how the 

 various groups of non-calcareous sponges are to be distributed along these 

 two lines of descent. 



The most primitive Demospongiae are to be found in the order 

 Carnosa, and particularly in the family Plakinidae. Plakina mono- 

 lopha is but little advanced beyond the primitive Ehagon type, 

 and its spicules are of small size, and for the most part of simple 

 tetraxon form. The progressive complications found in other 

 species of the same genus and in other genera of the Carnosa lead on 

 to the typical Tetractinellids Avith a Avell-developed cortex, and Avith 

 tetractines showing a corresponding differentiation betAveen three 

 tangential rays situated in the cortex and an enlarged ray 

 directed radially ; in other AA^ords, Avith triaenes. The Lithistida 

 are to be considered as developed from primitive Tetractinellids, in 

 Avhich some of the tetractines become encrusted and enveloped by 

 secondary deposits of silica to form the desma, with consequent 

 atrophy and reduction of the primitively tetraxon crepis, as Avell of 

 the free triaenes ; all stages of this evolution being still preserved in 

 different members of the group. 



