SPONGES 



133 



and one of the prongs lie in the plane of the field of vision, then the 

 angle between shaft and prong may vary greatly. The cladi may be 

 directed forwards, i.e. so as to point the opposite way to the shaft (protriaene, 

 Fig. 90, I); or outwards, at right angles to the shaft (orthotriaene, Fig! 

 90, ri) ; or even backwards (anatriaene, Fig. 90, k). In other words, each 

 cladus may rotate in the plane of the rhabdome, the amount of rotation 

 being always the same for each prong of a given triaene. 



In the second place, both the rays of the cladome and the rhabdome 

 may vary greatly in size relatively to one another, and any given ray may 

 become reduced until it finally disappears altogether. In the cladome the 

 process of atrophy, or rather arrest of development, may affect one ray 

 (diaene) or two of the rays (moncwne), or finally, all three, the result in 

 the latter case being a simple monaxon spicule (Fig. 90, j), a form of 



Fio. 90. 



Types of megascleres in Demospongiae. a-rf, rhabdi (a, strongyle, b, tylote, c, oxea, d, 

 tylotoxea) ; e-g, styli (e, tylostyle, /, style, g, spined tylostyle) ; A, branched monaxon ; j-o, 

 modifications of the triaene (j, cladi reduced, t, anatriaene, I, protriaene, m, orthotriaene, n, 

 dichotriaene, o, centrotriaene, p, amphitriaene, q, crepis of r, rhabdocrepid desma, 5, older and 

 fully formed desma. 



common occurrence in the Tetraxonida and known as a rhabdus (diactinal) 

 or style (monactinal). In cases where all the triaenes are reduced in this 

 way, the sponge may be entirely without tetraxon spicules, its Tetractinellid 

 affinities being shown only in secondary characters, such as the possession 

 of polyaxon microscleres or a cortex, and especially in the radiating arrange- 

 ment of the large monaxon spicules themselves, an orientation easily 

 intelligible on the assumption of their derivation from the rhabdome of & 

 triaene. Instances of such forms are well seen in the Placospongidae and 

 Tethyidae. On the other hand, the modification of the triaene may pro- 

 ceed along a course exactly opposite to that which produces a monaxon, 

 the rhabdome becoming atrophied and leaving the three rays of the 

 cladome as a triactinal spicule, usually situated close to the outer surface 

 of the sponge. 



As aberrant forms of the triaene may be mentioned finally the cases 



