THE SPONGES. 113 



Each flagellated chamber is provided with a special afferent as well as 

 with a special efferent canaliculus (Fig. 5, Plate 15). The two canaliculi 

 are of about the same size. On focussing, it may be seen that the posterior 

 wall of the chamber (boundary membrane) is perforated, the cavity of the 

 afferent canaliculus thus connecting with the set of spaces lying between 

 the collar cells. The chambers are 30-40 /a wide, and if the immediately 

 adjoining part of the efferent canaliculus be included, are pear-shaped. 



The chambers (Fig. 5, Plate 15) have a distinctly developed Sollas's 

 membrane, which is not very deeply concave. The collar cells, here as 

 in Poecillastra, are surprisingly few. They are often broken across, one 

 end clinging to the boundary membrane and one to Sollas's membrane. 

 They are frequently unbroken, however, and especially good ones may be 

 found in which the whole cell is doubly contoured, the lower half with 

 the nucleus granular, the upper part clear and expanding so as to embrace 

 a fenestra of Sollas's membrane. 



Megascleres. 



1. Dichotriaene, Figs. 9, 11, Plate 15. Rhabdome smooth, tapering to 

 point, 900 ju, X 140 fx. Cladi flattened parallel to surface of the sponge ; 

 smooth, broad, leaf-like, and irregular. Frequently one of a pair of 

 deuterocladi is reduced to a rounded protuberance. Less commonly one 

 or two of the protocladi may show no sign of bifurcation. Axial canals 

 distinct. Radius of cladome, from centre to apex of cladus, 700 ju, ; proto- 

 cladus, 190 /i, wide. 



The spiciiles form a single layer at the surface of the sponge, cladomes 

 tangential and supporting the dermal membrane. The cladomes overlap 

 to some extent and frequently interlock, and in general are so connected 

 together as to form a continuous dermal framework. The spicule ap- 

 pi'oaches the shape of the lithistid j^hyllotriaeno, e.g. in Discodermia (Sollas, 

 1888, Plate XXXIL). 



2. Oxea. Spicule smooth, tapering sufficiently to appear fusiform ; 

 slightly curved or sometimes about straight. A characteristic size 2300 /u, 



X 70 /.I. Present in parenchyma, singly, or more commonly in loose 



sheaves of 2 or 3 to about 6 spicules. For the most part arranged more or 

 less radially, often very obliquely to the surface, and in general not extend- 

 ing into the most superficial region, but stopping about where the rhabdomes 

 of the triaenes begin. In some cases they accompany the triaene rhabdomes. 

 The sheaves and single spicules are abundant, but not at all crowded. 



15 



