THE SPONGES. 107 



body (Fig. 1, Plate 15 ; Fig. 1, Plate 16), which is spheroidal, oval, or 

 rounded polygonal is about 20 fj. in diameter, and is filled with closely 

 packed spheres 3-4 fi in diameter. A nucleus can sometimes be made out. 

 The cells are identical with some of the " cellules spheruleuses " described 

 by Topsent (1894, p. 284; Fig. 18 s, Plate XVI.). 



The flagellated chambers (Fig. 1, Plate 16) are eurypylous, opening 

 by wide apertui-es into the efferent canals. The basal membrane on which 

 the collar cells rest is perforated by several chamber pores, which place 

 the set of spaces lying between the collar cells in communication with an 

 afferent canal (Fig. 1, Plate 16). The basal membrane in surface view 

 (Fig. 2, Plate 15) appears as a thin, finely granular membrane, which 

 shows no cell boundaries. On it rest the cell bodies of the collar cells, 

 appearing as angular masses each enclosing its nucleus. The preser- 

 vation is good, and the chambers show Sollas's membrane. This membrane 

 (Fig. 1, Plate 16) is distinctly fenestrated, and has an appearance sug- 

 gesting a firm, dense structure. The collar cells appear as shown in the 

 figure. The expanded lower part of the cell is granular, and contains the 

 nucleus. Above, it is prolonged into a transparent homogeneous process, 

 which at first is double-contoured, then for a short distance single-con- 

 toured, expanding and becoming double-contoured again where it passes 

 into Sollas's membrane. In the case of some cells it may be seen that this 

 process (doubtless the collapsed collar) passes into Sollas's membrane in 

 such a way as to bound one of the fenestrae. I am thus able to confirm 

 Sollas's observations (1888, pp. xxxvi-xxxvii), and may add that while 

 the appearance of the chambers may not be entirely normal, it seems 

 improbable that Sollas's membrane is an artefact. 



The chambers vary a good deal in size, but are often 40-50 jx wide • 

 frequently strongly flattened, the actual cavity of the chamber becoming 

 shallow. The wall of the chamber, from the basal membrane to Sollas's 

 membrane, is noticeably thick. 



Megascleres. 



1. Oxea. a. Fairly stout form, about 2.5 mm. x 40 /a; smooth, tapering 

 at each end ; commonly slightly curved or bent at the middle, sometimes 

 nearly straight. Abundant in choanosome ; here scattered in all directions, 

 usually singly, but also in loose tracts of two or three. In the superficial 

 region many oxeas are arranged radially or obliquely, often projecting 

 slightly (Fig. 1, Plate 15). Tangentially placed oxeas occur in consider- 



