44 f. F. .TENKIN. 



in 130 /t long x '.) n thick. Oral angle 155 to ICO . Apical ray sharply 

 pointed, 50 /A to 70 ^ long x LOyu, to lo/x thick, pointing considerably 

 upwards. Angle with basal ray 130. 



PART III. 

 HISTOLOGY. 



The specimens are not sufficiently well fixed to enable many histological details to 

 be made out satisfactorily, but as most of them are unique, it seems advisable to record 

 all that can be determined with reasonable certainty and accuracy. 



PORES, OSCULES, ETC. 



Several different methods are found among calcareous sponges for regulating the 

 circulation of water and preventing the entry of dirt or enemies. 



Ostia. The admission of water to the iucurrent canals is regulated in many 

 species by the opening and closing of the ostia. These pores are inter-cellular, and are 

 closed by the action of the surrounding cells. Fig. 48 shows a closed ostiuni at 

 the end of the short passage leading through the dermal cortex into the incurrent 

 chamber. The same passage is shown in tangential section in Fig. 49. 



Incurrent Canals. So far as the author is aware, there is no species in which 

 the incurrent canals close. 



PrOSOpyles. It is probable that the prosopyles can be closed in all species. 

 Considerable discussion has taken place as to whether prosopyles are inter- or iutra- 

 cellular. The explanation appears to be simple. The prosopyle is an opening in a 

 pore cell * (intra-cellular) which leads into the flagellated chamber between the 

 flagellated cells (inter-cellular). As, however, the flagellated cells are always much 

 more conspicuous in a surface view than the epithelial cells, the pore when seen in this 

 view presents the misleading appearance of being an inter-cellular opening. 



In most of the Heterocoela (excepting some species of Leucandra) the gelatinous 

 mesoderm is very little developed, so that the walls of the flagellated chambers appear 

 to consist of epithelium lined with flagellated cells. The pore cells are special cells in 

 this epithelium. In the H<>III<'<>< /</, where there is a certain thickness of mesoderm 

 containing the spicules, Minchin has shown (1) that the pore cells reach through this 

 to the level of the flagellated cells, thus forming pipes through the wall. In the 

 very thin walls of the flagellated chambers of most species of Leucandra the pore cells 

 have no appreciable length, the opening appearing as a hole through a thin membrane, 

 see Figs. 36, 37 and 38. 



In Leucandra gelatinosa, the gelatinous mesoderm is so much developed that the 

 flagellated chambers appear as spherical spaces lined with flagellated cells, and situated 



' Viilc Minchin (1). 



