xl THE VOYAGE OF H.M.S. CHALLENGER. 



downwards, between radiating brushes of sj)ieules, into one large inhalent canal (i.e.), 

 from which smaller canals or lacunse originate. Here again we find an intimate 

 connection between the arrangement of the pores and the nature of the dermal skeleton, 

 for the entire surface of the sponge, except on the tops of the raised pore-areas, 

 is covered with a dense, continuous armour of outwardly projecting discastra 

 (PI. LI. figs. l,d; lb, d), between which there is no room for the pores. We have 

 already seen how a similar arrangement of the pores, consequent upon the denseness of 

 the dermal skeleton (which is, however, very diff'erently arranged in the two cases, as will 

 be seen by comparison of PL XLVI. fig. 5, with PI. LI. fig. 16), in a totally different 

 sponge, viz., Halichondria latrunculioides, has given rise to a similar external form. 



We ought perhaps also to consider under this head one more case, viz., that of 

 Tentorium semisuberites, in which there is, however, only a single, large pore-area. The 

 arrangement of the dermal skeleton and ectosome in this sponge has been already referred 

 to (p. xxxi), the columnar body (PI. L. fig. 3) is encased in a quite impenetrable sheath of 

 spicules, except at the top (PL L. fig. 3, p.a), where the dermal skeleton is differently 

 (radiately) arranged. In the centre of the cushion-like top arises usually a single oscular 

 tube (PL L. fig. 3, o), and all around this are numerous pores, seen best in longitudinal 

 section (PL L. fig. 3a), leading into the elongated subdermal cavities between vertical 

 brushes of spicules, much as in other Suberitidse. There are no pores except on the 

 summit of the sponge. 



(2) Pores localised in areas in accordance with the arrangement of the subdermal 

 cavities; the dermal skeleton being either absent, or so feebly developed as not to interfere 

 with their arrangement. — This condition is found most frequently in non-corticate sponges, 

 and here, as before, it is impossible to say exactly where the scattered condition ceases 

 and localisation begins. We shall content ourselves with taking a few examples in 

 which the localisation is more or less distinctly marked. 



In the genus Myxilla it is exceedingly common to find the surface of the sponge 

 marked out into more or less regular, oval or rounded pore-areas, marking the position of 

 the underlying subdermal cavities, and this without any relation to the dermal skeleton, 

 which is commonly very vaguely and poorly developed ; as examples we may cite 

 Myxilla crihrigera, nobis, Myxilla mariana, var. massa, nobis, Myxilla nobilis, nobis, 

 and Myxilla compressa, nobis. Fig. 2a on PL XXX. shows this condition as seen in 

 Myxilla nobilis. For further details the reader is referred to the descriptions of the 

 several species mentioned. 



We also find this condition occurring in the genus Phahellia, e.g., Phahellia flabel- 

 lata, nobis, where the pores occur in smaU groups over the ends of narrow inhalent 

 canals ; and in Phahellia ventilabrum, var. connexiva, we have much the same thing 

 (vide PL XLIX. fig. 3). In neither of these cases is the skeleton arranged so as in 

 any way to determine the arrangement of the pores {cf p. 171, and PL XLIX. fig. 3). 



