134 SPONGES 



in which the rhabdome is prolonged beyond the cladome (centrotriaene, 

 Fig. 90, o), or bears a cladome at each extremity (amphitriaene, Fig 90, p), 

 and any of the varieties above mentioned of the tetractinal spicule, triaene, 

 or calthrops, may have one or more of its rays forked or branched like a 

 crest. The spicule is then said to be monolophous, dilophous, trilophous, 

 or tetralophous according to the number of rays so affected. When all 

 the rays are branched, the spicule may be termed simply a lophocalthrops 

 or lophotriaene. A special case of the latter is the candelabrum char- 

 acteristic of the Corticidae. Another common spicule, the dichotriaene 

 (Fig. 90, n\ has each cladus forked. 



(b) Polyaxon Type. The most primitive form of polyaxon spicule 

 is a simple globule or siliceous concretion which, by the acquisition 

 of numerous spines or rays, becomes an aster. The latter in its 

 turn undergoes numerous modifications, of which we may note in 

 the first place two series, in one of which the rays meet at a 

 common centre (euaster, Fig. 48, m, n\ while in the other the rays 

 are not centred, but radiate from a longer or shorter axis, usually 

 spiral (streptaster, Fig. 48, d, e). 



Further variation of each of these two sub-types gives rise to a great 

 number of forms. We may notice specially certain forms of systematic 

 importance, as, for example, the sterraster (Fig. 47, g), in which an aster 

 with numerous rays (in some cases apparently a euaster, in others a 

 streptaster) becomes converted secondarily into a solid spherule by deposits 

 of silica between the rays ; the spiraster, a streptaster with a spiral axis 

 (Fig. 48, d) ; the amphiaster, a streptaster with the rays confined to two 

 whorls at each end of the axis (Fig. 48, /) ; the samdaster (Fig. 48, e) ; 

 and the two modifications of the euaster, termed respectively oxyaster 

 and sphaeraster (Fig. 48, m, n). Of great morphological importance, on 

 the other hand, are the variations of the aster produced by reduction of 

 the rays (Fig. 48, o, p). Thus a euaster with only four persistent rays 

 becomes a microcalthrops (Fig. 48, p) or primitive tetraxon, which, by 

 curvature, branching, or ornamentation of the rays, gives rise to a large 

 series of microscleres, while increase of size makes it the starting-point 

 of the evolution, wholly or in part, of the megascleres. By a further 

 reduction of the rays of the euaster to two placed in the same straight 

 line, or, it may be, by suppression of the spines and elongation of the 

 axis, in a streptaster, we obtain a minute monaxon or microrliabdus, itself 

 the ancestor, so to speak, of many forms of microscleres, and perhaps of 

 megascleres ; of the former, the sigmaspire (Fig. 48, a, b\ perhaps 

 derived immediately from a spiraster by suppression of the rays, deserves 

 special mention. 



Secondary Spicules or Desmas. There remain for consideration 

 the remarkable megascleres known as desmas (" clones," Rauff), 

 characteristic of the sub-order Lithistida. Each desma is formed 

 typically by secondary deposits of silica upon a true spicule termed 

 the crepis or foundation, which undergoes an arrest of development. 



