SPONGES 



combined with a Greek numeral, as "monaxon," "triaxon," etc. 

 The number of rays present, on the other hand, is connoted in a 

 similar manner by substantives terminating in " actine," or by 

 adjectives terminating in "actinal," for example, "diactine," or 

 "diactinal spicule." The former series of terms is usually em- 

 ployed to express rather the ideal type of any given spicule, the 

 latter to describe its actual condition. 



The following types of spicuhe can be recognised in sponges 

 generally, each type exhibiting in its turn innumerable variations : 



(1) The monaxon type of spicule, built upon a single axis, and 

 having therefore simply the form of a rod or needle (Fig. 47, e^and 

 b). A monaxon spicule may be either monactinal (Fig. 47, b) or 



f\ 



a 



FIG. 47. 



Types of spicules (megascleres). a, rhabdus (diactinal monaxon) ; 6, stylus (monactinal 

 monaxon) ; c, triactine ; d, tetractine (tetraxon type) ; e, hexactine ; /, desma of an anomocladine 

 Lithistid (secondarily polyaxon) ; g, sterraster (polyaxon) ; h, radial section through the outer 

 part of g, showing two actines soldered together by intervening silica, the free ends terminating 

 in recurved spines, and the axis traversed by a central fibre. 



diactinal (Fig. 47, a), the two rays in the latter case being placed 

 in the same straight line. The axis may be straight or curved 

 (Fig. 48, a, b, c, etc.). 



(2) The triaxon type, characteristic of Hexactinellids (Fig. 47, e). 

 The primitive spicule is laid down along three axes which cut one 

 another at right angles at a central point, producing a six-rayed or 

 hexactinal spicule, which may undergo a secondary reduction of the 

 rays ; but so long as more than one ray persists, it meets its fellow 

 or fellows at angles of 90 or 180. 



(3) The tetraxon type of spicule (Fig. 47, d), which may be con- 

 sidered ideally as laid down along four radii of a sphere which meet 

 one another at equal angles at the centre. Hence the primitive 

 form is a tetractine, of which any three rays will appear to meet at 

 angles of 120, when projected in such a way that the fourth ray 



