142 Prof. W. J. Sollas on the 



1. Tlie Cortex. — (i) The outermost layer of tlie cortex is 

 an exceedingly thin, colourless, and transparent membrane, 

 which rests on a layer of bacillar spicules, homologous with 

 the layer of minute stellates in other Geodine sponges. 



(ii) The bacillar layer is single, i. e. only one bacillus 

 thick, the bacilli lying parallel to its surface, orientated in 

 every direction, in close contact with each other. Since there 

 is nothing intervening between the superficial membrane and 

 the bacillar layer, it is clear that the former must be the 

 ectoderm, if the generalization hold good that all the skeletal 

 parts of the sponge originate in the mesoderm. That it now 

 exhibits no ectodermic structure is in full accordance with 

 Schulze's observations, who states that he has never yet seen 

 the characteristic platten-cell outlines in spirit-specimens, but 

 only in fresh specimens under silver or gold treatment. 



(iii) The succeeding dermal layer is described by Bower- 

 bank * as "a stratum of membranous structure and sarcode 

 destitute of gemmules (globates)." It is of very variable 

 thickness, in some places 0'0038 to 0*0075 inch across, in 

 others absent, the underlying globates then coming in contact 

 with the bacillar layer ; it closely resembles the vesicular 

 or vacuolated connective tissue of Geodia Barretti (see antehy 

 vol. V. p. 251) ; in some parts it consists simply of separate 

 colourless, transparent, more or less oval, or polygonal cells 

 with remarkably thin walls and devoid of contents, except for 

 a very small quantity of colourless sarcode, in which may 

 usually be detected a nucleus with its nucleolus (PI. VI. figs. 5 

 and 13) ; in other places, however, the tissue exhibits in 

 addition an intermediate substance, which joins the adjacent 

 cells together, and appears to result from the metamorphosis 

 and fusion of their walls ; the intermediate substance is 

 usually colourless and structureless ; but sometimes it assumes 

 a dusky grey tint, owing to the presence of abundant minute 

 granules ; it also appears in some cases to become finely 

 fibrillated (PI. VI. fig. 5). 



(iv) The globate layer, 0'03 inch thick, has the same 

 structure as in other Geodine sponges. The ligaments which 

 join adjacent globates together consist of fine structureless 

 parallel fibrilla3, amongst which at intervals occur parallel- 

 lying granular threads, sometimes containing a nucleus and 

 nucleolus (PI. VI. fig. 11). They are probably derived, like 

 the connective-tissue fibres, from elongated fusiform cells with 

 hyaline walls and granular axial threads, in which the hyaline 

 walls have become completely fibrillated, while the axial 

 thread remains unchanged. 



* Phil. Trans. 1862. 



