458 BEVISION OF THE AXINELLID^^ i., 



and T. reteporosus: in the former, strongyla are extremely rare, 

 and the megascleres are ahnost exclusively sharp-pointed, fusiform 

 oxea, attaining a maximum size of 530 by 27 /z; in the latter, 

 strongyla and oxea are about equally numerous, the oxea are 

 mostly more or less blunt-pointed and but very slightly fusiform, 

 and their maxinnnn size usually does not exceed 300 by 8/x. In 

 most of the species, a certain proportion of the megasclei'es (ap- 

 parently those alone which occur extra-fibrally in the axial region 

 of the skeleton) are found to attain an increasingly larger size as 

 one proceeds towards the older portions of the sponge, with the 

 result that, in the stalk, the maxinnnn size of the megascleres is 

 notably greater than in the uppermost parts of the branches; and 

 these largest spicules, even in the species in which strongyla 

 abound, are almost without exception oxea. The spicules of the 

 fibres are no larger in the stalk than elsewhere. 



The spinispirulaj are mirmte, entirely spinulous, for the most 

 part regularly corkscrew-shaped spicules, rarely of more than two 

 complete turns; in addition, they comprise a series of simpler 

 forms, of various shapes ranging from that of a much contort §, 

 through C «baped forms, to straight or nearly straight rods (Text- 

 fig. 3). The proportionate number of these simpler forms varies in 

 the different species^ but the degree of variability in this respect, 

 as well as in other characters of the spirulaj, is not sufficient to 

 be of diagnostic value. An exception to this rule, however, is 

 possibly afforded by the spirulaj of T. kevisjnridi/er, which have 

 been described by Carter as smooth; but it is more probable that 

 the spicules, in this case, were not examined under a sufficiently 

 high power to render their spination visible. 



The microstrongyla are inconstant in occurrence, and they 

 may be either ninuerous or scarce, or perhaps sometimes entirely 

 absent, in difi'erent specimens of the same species; at any lute, 

 this was found to be the case in T. dvjitahis (typical variety), 

 and T. reteporosus (var. ?) — of which alone a number of specimens 

 were available for examination. That they are proper spicules, 

 however, and not merely pathological products, is rendered certain 

 by their degree of uniformity in size and shape. Occasional 



