SPONGES 



137 



stony hardness. This mode of union of the spicules is termed 

 " zygosis." 



(/3) Monax&nida. The skeleton of the Monaxonida is composed 

 of siliceous spicules, to which may be added a greater or less 

 amount of spongin. The function of the latter is, in the first in- 

 stance, that of a special cement, which glues the spicules together, 

 but it may be present in such quantities that it forms the greater 

 part of the skeleton, especially in forms whose habitat exposes them 

 to severe stresses and strains from waves and currents (Keller). 

 Hence the spicules are thrown more and more into the background, 

 and tend to become reduced and rudimentary. In any case, the 

 spicules of Monaxonida are, as a general rule, smaller relatively to 

 the size of the sponge than is the case in Hexactinellids and 

 Tetractinellids, and in order to support the sponge adequately, 

 they tend to become united to form more or less definite tracts of 

 fibres, a type of skeleton which has the further advantage of pos- 

 sessing the flexibility and elasticity essential to a shore life. 



The formation of a skeletal framework by union of spicules, 

 permits of a sharp distinction being drawn, as a rule, between 

 megascleres and microscleres, since the former enter into the com- 

 position of the body skeleton (skeletal spicules), while the latter 

 are scattered in the tissues (flesh spicules). In some cases, how- 

 ever, the distinction is one of degree and scarcely tenable, as in the 

 Spongillinae. In many cases microscleres may be wanting entirely. 



Forms of Spicules. All spicules in this group are either of the 

 monaxon type, or in a few cases among the microscleres, polyaxon. 

 Since, however, monaxon spicules are of frequent occurrence in 

 other groups as reductions of triaxon and tetraxon types, it is not 

 so much the presence of monaxons, as the absence of other types, 

 which specially characterise the Monaxonida. 



(a) The megascleres are always monaxon, and their variations, 

 though numerous, are within a small compass. The most import- 

 ant distinction that can be drawn depends upon the spicule being 

 monactinal (styli, Fig. 90, e, /, g\ or diactinal (rhabdi, Fig. 90, a-d). 

 In the former case, the slight swelling in the axial thread that 

 marks the starting-point of the growth is near one extremity, 

 which may be termed the proximal end of the spicule ; in the 

 latter case, it is near the middle of the shaft. Monactinal spicules 

 always have the two ends unlike, the proximal end being rounded 

 off abruptly, and often knobbed ("tylostyle"). Diactinal spicules, 

 on the other hand, usually have the two extremities similar. 



Other variations in the monaxon spicule, apart from fluctuations of 

 size, depend on whether the shaft is smooth or spined, straight or curved, 

 or whether the extremities are sharp ("oxeote"), blunt ("tornote"), 

 rounded (" strongylote "), knobbed (" tylote "), or, in rare cases, branched. 

 The branching is probably due, in most cases, to the development of 



