!24 POR1FERA. II. 



compressed and have sharp edges, these edges, however, are most frequently finely spined, which is 

 owing to the skeleton below. The dermal membrane is a rather thin, transparent film. It is supported 

 by bundles of dermal spicules projecting in a fan-shaped way. These spicula bundles, however, are 

 frequently not perpendicularly erect, but more or less, often highly recumbent. Of spicules the mem- 

 brane otherwise has only microscleres. In different individuals, or in different places of one individual, 

 it may have a somewhat different appearance; in more compact individuals with not especially large 

 canals (perhaps more contracted individuals), or in places of the individual showing such a condition, 

 the skin is rather smooth, and the projecting spicula-bundles are here close-set and rather perpendicular. 

 Where the surface is more grooved and the membrane stretched over the grooves, the structure is some- 

 what different, as there are here large parts of the membrane with no skeleton immediately below them, 

 from which the bundles ma}- arise. Then the bundles issuing in a fan-shaped way from the edge reach 

 into the membrane, and are quite, or almost quite recumbent in it, and in the middle part of it, where 

 the bundles do not reach, scattered horizontal spicules are further found. In other places the structure 

 is again somewhat different, fibres running from the skeleton under the parts of the membrane 

 stretched over the grooves, from which fibres fan-shaped bundles project and pass into the membrane 

 where they are more or less horizontal. This latter structure gives to the dermal membrane a peculiar 

 appearance, as the parts of the membrane that are stretched over the canals or the subdermal cavities 

 and, on account of the cavities below, appear as dark areas, are again subdivided by the mentioned 

 fibres, which form a reticulation, in the meshes of which the pores are then situated. The pores are 

 found in the dermal skeleton in the areas formed by the skeleton; they may be present every where, 

 but are seen especially numerous and close-lying in the parts of the dermal membrane situated over 

 the subdermal cavities, which parts are here often reduced to a sieve. They are round or oval and 

 of sizes from quite small ones to cri5 mm . Oscula are found scattered in rather slight numbers; as men- 

 tioned above, they are sometimes found on the top of conical projections, but these projections are 

 often quite low or quite wanting. In a few cases the conical projections are not separated, but form 

 a continuous wall with several oscula placed in the edge. This structure is illustrated by Bower- 

 bank's figure PI. XLIV, 7. In the largest specimen in hand eleven oscula are found. The appear- 

 ance of the oscula may be somewhat varying; in the more compact forms with even surface the 

 osculum is a regular round or oval aperture definitely bounded by the skin, which rises sometimes 

 to a sharp edge round the aperture. Here the oscula are most frequently small, and were measured 

 down to i mm or less. In individuals with highly grooved surface, on the other hand, the edge of 

 the oscula becomes often irregularly indented, so that the aperture is somewhat lobed. It is this 

 structure which causes Johnston to use the expression: ^oscula obscure, substellated , presumably 

 the dermal membrane has also been wanting to a great extent, and then oscula are little marked 

 compared to the openings of the inhalent canals. When oscula are of the last-mentioned structure, 

 they are generally rather large, and they were measured up to a diameter of io mm . 



The skeleton. The dermal skeleton consists, as already mentioned, of perpendicular or more 

 or less recumbent bundles of dermal spicules, which pass off from the main skeleton and support the 

 dermal membrane, and bend as fibres under and into the parts of the membrane stretched over the 

 subdermal cavities. Generally the question is only of bundles; but sometimes, in certain places of the 



