134 SPONGES 



in which the rhabdome is prolonged beyond the cladorae {centrotriaene, 

 Fi<,'. 00, o), or bears a cladonie at each extremity {amphitriaoie, Fig 90, jj), 

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

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

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

 or tetralojihous according to the number of rays so aft'ected. "Wlien all 

 tlie rays are branched, the spicule may be termed simply a lojihocalthrojjs 

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

 acteristic of the Corticidae. Another common spicule, the dicliotriaene 

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



(//) Pulijaxon Type. — The most primitive form of polyaxon spicule 

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

 of numerous spines or raj^s, becomes an UKtrr. 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 {strejytastei; 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 sterrastcr (Fig. 47, r/), 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 ampJiiadcr, a streptaster with the rays cunfined to two 

 wliorls at each end of the axis (Fig. 48,/) ; the sanidaster (Fig. 48, e) ; 

 and the two modifications of the euaster, termed respectively o.njaster 

 and spltaemster (Fig. 48, m, v). Of great morphological importance, on 

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

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

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

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

 series of micruscleres, 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 microrhahduit, itself 

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

 megascleres ; of the former, the siginasjiire (Fig. 48, a, b), perhaps 

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

 special mention. 



Seconilari/ Sjnndcs or Desmas. — There remain for consideration 

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

 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. 



