SPONGES. 207 



while at the base of the sponge they are seen to originate in a thin horizontal spongin- 

 lamella, which evidently forms the means of attachment to the substratum. The 

 primary fibres are about 0'096 millim. in diameter, the secondaries are usually a good 

 deal more slender, say about 0'04 millim. in diameter, but variable. Both primary 

 and secondary fibres frequently exhibit a very distinct pith of variable thickness, 

 which evidently simply represents the older part of the fibre surrounded by fresh 

 accretions of spongin. Even in old parts of the primary fibres the old slender apices 

 may frequently be observed thus imbedded in the new growth (Plate XV., fig. 3, ap.). 

 The principal secondary fibres run across at right angles between the primaries, but 

 numerous others run in various directions. The outermost secondary fibres form a 

 pretty definite dermal skeleton with polygonal meshes of very variable size. Here 

 and there short, vertical, gradually tapering branches arise from this network and 

 enter some of the surface conuli. These branches evidently form the commencements 

 of new primary fibres, as in M. nigra. 



The ectosome is feebly developed, forming a thin dermal membrane containing the 

 inhalant pores and overlying the sub-dermal cavities. The canal-system is lacunar. 

 The flagellate chambers are large and sac-shaped, about 0'088 millim. in longer 

 diameter ; they open by wide mouths direct into wide exhalant lacunae. The ground 

 substance between them is very feebly developed, gelatinous-looking and broken up 

 into trabeculse by the smaller inhalant lacunae in the characteristic aplysillid fashion. 

 The larger exhalant canals converge towards the compound vents, where they open 

 to the exterior. 



It will be seen from the above description that this species agrees closely in 

 skeletal characters and canal-system with M. nigra, but it differs widely in external 

 form and colour (in spirit). The fact that the specimen contains large ova and 

 embryos in endothelial capsules likewise seems to indicate that it is not merely a 

 young form of M. nigra. Owing to the much greater distinctness of the pith in the 

 horny fibres the species is a more typical aplysillid than its congener. 



RN. 191 (Muttuvaratu Paar, Gulf of Manaar). 



FAMILY: SPONGELIID^E. 



Euceratosa with a (usually) reticulate skeleton of horny fibres without distinct pih, 

 but containing foreign bodies ; or with a skeleton composed of foreign bodies 

 united together by little if any spongin. With lacunar canal-system and large, 

 sac-shaped flagellate chambers opening directly by wide mouths into wide 

 exhalant lacunae. 



This family may be retained as a matter of convenience, but it is, as I have 

 already indicated, logically impossible to separate it sharply from the Aplysillidse, for 

 the genus Megalopastas, on the one hand, and SCHULZE'S Spongelia spinifera, on the 

 other, are strictly intermediate between the two groups. 



