402 Mr. W. J. Sollas on the 



contained spherical nucleolus. By treating with a 5-per-cent. 

 solution of caustic potash, the cell-wall expands and separates 

 from the contained globates completely (PI. XVII. fig. 19). 

 With age a hilum is formed, as previously described in the 

 case of G. Barretti) but the nucleus merely occupies and does 

 not completely fill the hilum (PI. XVII. fig. 18) , as errone- 

 ously stated in the previous description (Ann. & Mag. Nat. 

 Hist. ser. 5, vol. v. p. 256). The external ends of the trichites 

 grow much thicker with age, and assume a sharp conical 

 form ; the sharp ends of the conical spines then become 

 rounded off and pass into rounded conical tubercles ; these 

 finally become flattened and spined round the summit, and 

 the globate is complete. Absorption next ensues. The adult 

 globate always exhibits in section a small central cavity 

 with fine radiate canals proceeding from it ; the effect of 

 absorption is to enlarge this cavity and its radiate canals, so 

 that the globate becomes eventually a mere thick-walled shell, 

 its walls being perforated by radiate canals of wide diameter 

 which extend along the axes of the exterior tubercles, and 

 almost but not quite open to the exterior (PI. XVII. fig. 24) . 

 It appears that the layer of tubercles is liable to separate as 

 a thin shell from the rest of the globate spicule. 



Stellates. — The same kind of distribution of the stellates 

 occurs here as in G. Barretti and S. Normani ; none but the 

 sphaero-stellates occur immediately beneath the epidermis ; 

 elsewhere the second kind of stellates (PI. XVII. fig. 13) are 

 chiefly found ; the sphasro-stellates occur in the mark imme- 

 diately beneath the rind, but less abundantly than the other 

 forms. The small-bodied stellates occur lining the interior of 

 the sphinctral canal, in the subcortical layer, and generally 

 through the mark, but especially in the wall of the water- 

 canals. 



4. The Water-canal System. — The characters of the incur- 

 rent and excurrent ostial tubes have already been referred to ; 

 as they appear to represent the chones of other Corticatse, it 

 will be convenient to distinguish them as incurrent and ex- 

 current chones. The two kinds of chones differ only in size, 

 both being freely open distally and closed below by a sphinc- 

 ter, which protrudes downwards into a canal which here re- 

 presents the crypt (PI. XVII. figs. 1, 3, 20). Thus there is 

 no endochone, and the chones are the equivalents of the 

 ectochone alone. 



The incurrent chone leads into a canal which extends 

 parallel to the surface just below the rind for a variable dis- 

 tance, giving off one or more branches, which descend radiately 

 into the interior, and break up into still smaller canals. 



