REPORT ON THE MON AXON IDA. XXXVii 



is no separable dermal membrane, the pores must usually be sought for in sections taken 

 at right angles to the surface. Whether the arrangement of the pores is primarily 

 dependent upon the arrangement of the subdermal cavities or vice versa, or whether both 

 are dependent upon the arrangement of the dermal skeleton, is a very difficult question to 

 decide. Probably the arrangement of the dermal skeleton, which is of great importance 

 for the jjrotection of the sponge, to a large extent determines the arrangement both of the 

 pores and of the subdermal cavities ; we shall, later on, give strong reasons for beheving 

 this to be the case. In the meantime it is sufficient to remark that aU three are most 

 intimately correlated with one another. In the first place, it is obvious that the pores, in 

 order to fulfil their function of admitting water into the sponge, must always be 

 placed over the subdermal cavities, from which the inhalent canals take their origin. 

 This, together with the arrangement of the dermal skeleton, usually results in a 

 more or less regular grouping of the pores in "pore-areas." Sometimes, however, 

 it is impossible to detect any regular grouping of the pores whatever ; they are 

 simply scattered over the surface, here and there. We have thus two principal 

 types of pore-arrangement to distinguish — the pores may be scattered, or they may 

 be collected in more or less definite "pore-areas." We shall consider these two cases 

 separately. 



(o) Pores scattered, not collected in definite Pore-areas. 



It is not very often that we get the pores quite irregularly scattered, for usually they 

 are constrained to group themselves more or less regularly, either by the exigencies of the 

 subdermal cavities or of the dermal skeleton. Still, in cases where there is no dermal 

 skeleton present, or where this is of such a nature as not to interfere with their 

 arrangement, we sometimes find this condition occurring; as for example, in Petrosia 

 his2nda (no dermal reticulation), Reniera suhglobosa (dermal skeleton unispicular), 

 Vom'erula esperioides (dermal skeleton reticulate, but meshes very wide), Esperella 

 mammiformis (dermal skeleton loosely reticulate), Esperella lapidiformis (dermal 

 skeleton absent), Esperella simonis (dermal skeleton irregularly reticulate), Desmacidon 

 (Homosodictya) kerguelenensis (skeleton irregular, very loose), Myxilla rosacea, var. 

 japonica (dermal skeleton radiate), Myxilla frondosa (dermal skeleton loose and irregular), 

 and most Suberitidse (dermal skeleton radiate). But here, as in so many other 

 cases, we can draw no hard and fast line, and in many cases it is impossible to 

 decide whether the pores should be classed as irregularly scattered or collected in 

 areas. Perhaps the best cases of irregularly scattered pores are to be found in the 

 Suberitidse, e.g., Stylocordyla stipitata, var. glohosa, where the dermal skeleton is 

 radiately arranged and therefore does not interfere with the arrangement of the pores 

 (PI. L. fig. 1, 25.). 



