SPONGES.-HALLMANN. 



135 



disposed large oxea. The sub-cortical zone is crossed at close 

 intervals by slightly penicillate fascicles of similar spicules, 

 which give support to the cortical layer. The innermost 

 spicules of these bundles emerge from between the spicules of 

 the core ; the outermost project for a considerable distance 

 beyond the surface. There are thus three orders of projecting 

 spicules at the surface. The inter-fascicular spaces of the 

 subcortical zone are filled-in with soft tissues containing 

 scattered spicules and traversed by canals which in cross- 

 section show a circular outline and a surrounding whorl of 

 radially-disposed microxea. The largest canals occur in the 

 deeper parts of the zone, and may attain a diameter of .6 mm. 

 In the immediate neighbourhood of the cortex, the canals are 

 of capillary dimensions, and the microxea — which presumably 

 surround them — are so closely and confusedly intermingled, 

 that the precise inner limit of the cortex is very often difficult 

 of determination. 



Megascleres. — 



(i.) The oxea of the inner skeleton are straight or only 

 slightly curved symmetrical fusiform spicules attain- 

 ing a size of 3200 x 50 j<. They are usually much more 

 than 1000 )i long, but spicules of all lengths between 

 these larger oxea and the dermal oxea occur, and 

 (since the asymmetry of the latter is sometimes in- 

 appreciable) consequently no actual lower limit can 

 be assigned to their size. Abnormalities of these 

 spicules in the shape of styli of only about one-half 

 their length are of extremely rare occurrence. 



(ii.) The fusiform styli and (usually asymmetrical) oxea, 

 which form the dermal brushes and are also to be 

 found in small number scattered in the subcortical 

 zone, range in length from somewhat less than 200 

 to slightly more than 700 ;< ; the largest may attain 

 a diameter of 18 ji. All intermediate grades of form 

 between styli and oxea occur. 



Microscleres. — 



(i.) 'Ihe microxea are similar in form to the largest oxea, 

 but are about twice as stout in proportion to their 

 length. They are rarely less than 100 ji, or more 

 than 140 n long, and attain a diameter of slightly 

 more than 4 ji. Occasionally spicules intermediate 

 in size between these and the shortest dermal oxea 

 are met with, but appear to belong rather to the latter 

 category. 



