132 SPONGES 



Tetraxonida may be considered ideally — that is to say, from a 

 imrely architectural or geometrical point of view, and without 

 prejudice to the question of their actual phylogeny and evolution 

 — as modifications of one of two types: (a) the tetraxan type, 

 characteristic of the megascleres, though not confined to them • 

 and {h) the poli/uxon type, only found among the microscleres. 

 Strictly speaking, the tetraxon type itself could be considered as 

 a modification of the polyaxon, and has probably been derived 

 from it, but for practical purposes it is best to consider the two 

 types separately. 



(a) Tetraxon Type. — The simplest form of tetraxon spicule has four 

 equal and similar rays meeting at equal angles (Fig. 47, d and ;>). 

 Such a spicule is known as a caWtrops, and though of common 

 occurrence, both among megascleres and microscleres, it is far less 

 abundant than some of the numerous variations of the regular 

 tetraxon form. Departures from the fundamental type are brought 

 about, not only as in the Hexactinellida, by unequal growth or 

 curvature of the rays, or by the acquisition of secondary spines and 

 branches, but also, in contrast to the modifications of the triaxon 

 type, by variations in the angles at which the rays meet. 



The simplest modification of the regular tetractine is one 

 correlated in the first instance with the acquisition by.it of a 

 definite orientation in the sponge body. One ray, which is directed 

 radially and points towards the interior of the sponge, becomes 

 diflerentiated from the three remaining rays, which in their 

 turn radiate more or less tangentially from a centre situated 

 close to tlie outer surface of the sponge. In this way arises the 

 form of spicule known as the triaene (Fig. 90, k\ I, m, ii), which is 

 perhaps more than any other characteristic of the order Tetractinel- 

 lida. The radially directed ray of the triaene, which is usually 

 longer, but sometimes shorter, than the other three, is termed 

 th(i shaft or rhaMome, and the superficially situated rays are known 

 individually as the chali or prongs, collectively as the cladovie. 



The triaene undergoes in its turn numerous niodiliwvtions, aflecting 

 every part of it, and giving rise to a series of forms, each denoted by a 

 special term. Without attempting to enumerate the many varieties of 

 the triaene, it is of interest to consider the variations of the cladi in tlieir 

 relations to the rliabdome, both as regards orientation and size. 



In the first place, the three cladi or tlieir axes always meet one anotlier 

 at equal angles, but the angles at which tliey meet the rliabdome may 

 vary considerably in different instances, though always the sjime for each 

 cladus in a given .si)icule. Hence, if a pn.jection be made of the triaene 

 in such a way that tlie». shaft is completely foreshortened and seen 

 as a dot, then the axes of the three cladi, or of their main stems, if they 

 be branched, will appear to meet one anotlier at equal angles of 120°. 

 If the triaene be viewed in profile, on the other hand, so that the shaft 



