144 Prof. W. J. Sollas on the 



by Bowerbank *^ and afterwards more fully by Carter f. My 

 own observations, made on thin slices mounted some in gly- 

 cerine and some in balsam, all showing the structure in the 

 clearest possible manner, are different from those of Carter in 

 several particulars, and accord with those made by me on 

 Geodia BarreUi. A transverse section of the rind, giving a 

 longitudinal section of the chones, is represented in fig. 3, 

 PI. VI. The chonal roof consists of the epidermis and bacil- 

 lar layer above, bearing the pores : beneath follows the 

 vacuolated tissue, with interspersed fibrous elements concen- 

 trically surrounding the pore-canals, which descend one from 

 each pore. The pores in the centre of the roof lead directly 

 into the chone, those at its sides into lateral canals, Avliich may 

 be regarded as an extension laterally of the main chonal 

 cavity above the globate layer and beneath or through the 

 dermis. They are best exposed in tangential sections of the 

 chones. There is no trace of small independent canals tra- 

 versing the crust outside the chones. The ectochone has the 

 form of a truncated cone, the base being directed outwards ; 

 it is provided below with a well-developed sphinctral muscle, 

 the origin of which is about on a level with the inner face of 

 the globate layer ; the endochone has almost or entirely dis- 

 ap])cared, and the subcortical crypt is of very variable size 

 and irregular form. 



The epidermis and its associated bacillar layer are con- 

 tinued from the pores inwards, lining the poral canals and the 

 whole cavity of the chone ; they extend through the aperture 

 of the sphincter (the bacilli becoming very rare here), and 

 cover the walls of the subcortical crypt. Beneath the bacil- 

 lar layer in the walls of the ectochone is a layer of dermal 

 vacuolated tissue, about 0*002 inch thick ; it lies immediately 

 on the globate layer. 



The chonal sphincter varies in thickness according to its 

 degree of contraction ; when fully closed, its lower side has a 

 mamillary form and projects into the cavity of the subcor- 

 tical crypt ; this swollen protuberance may have given rise to 

 the notion of a spiral tube descending from the sphincter, 

 which does not really exist. It is covered by small roundish 

 cells, which are most clearly seen at its central margin, and 

 which are, without doubt, ectodermal. It consists chiefly of 

 dark granular muscle-fibres, which stain deeply with car- 

 mine ; they have a concentric and radiate arrangement, but 

 are mostly concentric. 



(ii) The Excun^ent CmiaJs. The same differences as dis- 



* Brit. Spong. vol. i. p. 101. 



t Ann. & Mag. Nat. Hist. 18G9, vol. iv. p. 13. 



