SPONGES 135 



The crepis may be a minute calthrops, or a rhabdus, or, finally, may 

 be atrophied completely ; thus tetracrepid, monocrepid, and acrepid 

 desmas may be distinguished. The layers of silica deposited are at 

 first concentric with the crepis, but subsequently grow out into 

 irregular branches and tubercles, which are quite independent of it. 

 In this way a secondary skeletal element of complicated and often 

 quite irregular form is produced (Fig. 47, / ; Fig. 90, q, r, s). 



Phylogeny of the Spicules. Enough has been said to indicate the 

 probable origin of the primitive tetraxon from the polyaxon aster or 

 globule, and hence the origin of all megascleres from the microscleres. 

 The regular tetraxon type of spicule represents an adaptation to the 

 structure of a primitive Rhagon-like ancestor, in which, by folding of the 

 walls, numerous spherical ciliated chambers lie embedded in a parenchyma- 

 tous tissue (Schulze). When in such a form, the chambers are as closely 

 packed as possible ; each chamber is in contact with three others, and 

 the tetraxon spicule fits exactly into the interspaces between four con- 

 tiguous chambers. 



The evolution of many of the forms of spicules is difficult to follow 

 in detail, since in many cases more than one origin is possible for them, 

 and not enough is known to determine with certainty which was the 

 actual course of the phylogeny, which may indeed have proceeded along 

 more than one direction. Thus in the case of the characteristic triaenes : 

 while, on the one hand, a general comparative survey of their morphology 

 and systematic relations rather indicates an origin for them from the 

 primitive tetraxon calthrops, correlated with the acquisition by the sponge 

 of a distinct cortex ; on the other hand, their ontogeny, so far as it is 

 known, and also the existence of certain forms such as the mesotriaene 

 and amphitriaene, favours the view that they have originated by branch- 

 ing of a large monaxon rhabdus (Sollas). Conversely, a double origin is 

 possible for the monaxon megascleres, either by reduction from a triaene, 

 or, by increase of size, from a microrhabdus, derived in its turn from 

 reduction of an aster or a calthrops. 



The following scheme may serve to indicate the different courses of 

 phylogeny which are possible : 



Primitive ^j -> euaster -> calthrops ->- triaene 



Globule -f 4" 4 ^ 



or Concretion J ->- streptaster ->- microrhabd. ->- macrorhabd. 



Arrangement of the Spicules in the Skeleton. By the arrangement 

 of the megascleres two types of skeleton can be distinguished in the 

 order Tetractinellida : the irregular, seen in the Lithistida, and a few 

 Choristida; and the radiate type (Fig. 91), characteristic of the vast 

 majority of Choristida. Even in the former type, however, all 

 triaenes when present near the surface have the rhabdome directed 

 towards the centre, and to this extent exhibit a radiate structure. 



In most, if not all, Choristida the young sponge has a radiate structure 

 when still quite small, the spicules being arranged in sheaves between 



