456 JANE STEPHENS: ATLANTIC SPONGES 



surface or may be slightly raised above it. The dermis is thin and is pierced by 

 numerous pores ; those measured varied between "025-0 "1 mm. in diameter. The 

 consistence is rather soft and compressible. 



The colour in spirit is pale greyish yellow, the younger sponges being lighter 

 in colour than the large ones. 



The main skeleton consists of a regular reticulation of fibres. The principal 

 fibres run upwards to the surface ; they pierce the dermis and project as long tufts 

 of spicules. They are about 0'03-0'075 mm. in thickness, and are distant from each 

 other about the length of one skeletal spicule, or, in other words, they are placed 

 on an average 0'2 mm. apart. They consist of oxea, multiserially arranged and 

 cemented together by spongin which does not form a distinct sheath. They are 

 connected by transverse fibres perpendicular to them, which run from one main fibre 

 to the next, but which do not themselves form continuous fibres. The transverse 

 fibres consist of single oxea, or of a bundle of several oxea more or less enclosed in 

 spongin. They are about 0'025-0'05 mm. thick, and towards the surface of the 

 sponge they lie nearer together than at the centre. Spongin is much more abundant 

 in some specimens than in others. There is no special dermal skeleton. 



Spicules. — (l) Oxea. — -These are slightly curved, tapering evenly to sharp points. 

 They vary between 0"l75-0'255 mm. in length, and have a maximum diameter of 

 0'013 mm. In the majority of the large specimens they are between '2-0 "25 mm. 

 long, but in a few sponges they are only 0'175-0'225 mm. in length. In the 

 youngest specimens the oxea are shorter, but they increase in length, on the whole, 

 with the size of the specimen, until the maximum length is reached. The greatest 

 thickness of the oxea is not much less than that of the oxea in the adult specimens, 

 being about O'Ol mm., but many very slender oxea are to be seen. The shorter oxea 

 are rather sharply bent in the middle. The curve becomes less abrupt, as a rule, 

 the longer the spicule. In the embryos the oxea vary from 0'075 mm. in length to 

 0'2 mm., and from exceedingly slender to about 0"006 mm. in thickness. (2) Isochelse 

 palmatse. — These are scattered through the sponge. In some specimens they are 

 fairly numerous, in others a few only can be found after prolonged searching. 

 Typically they have the end of the axis bent out from the inner side of the tooth, 

 but often this structure cannot be made out. The isochelse are about 0'032-0'04 mm. 

 in length, and about O'Ol mm. across the tooth. In the young specimens the 

 isochelse are usually '032-0 '035 mm. long, and they appear to be relatively more 

 abundant than in the older specimens. The bending of the axis from the inner side 

 of the tooth is usually clearly seen in them. No isochelse were found in any of 

 the embryos. 



Some of the specimens, as already mentioned, are very like Homceodictya 

 palmata (Johnston) in external appearance, and the arrangement of the skeleton 

 is very similar in the two species. On the other hand, Homceodictya multiformis 

 possesses rather longer and more slender oxea. The isochelse, too, are larger, and 



