PORIFERA. III. 



into two groups of sizes, large and small; the dermal sficziles are as a rule diactinal, generally strongyla, 

 sometimes tylota, tornota or oxca; in a few cases they are monactinal, styli. Microsclera are chelce 

 arcuata, solely or together zvith sigmata, rarely sigmata alone, in a single case rhaphidrs alone; some- 

 times there are no microsclera. 



The species of Hymedesmia are all incrusting forms, as it is a generic character, that the 

 skeletal spicules are all based on the substratum; the species are therefore all of insignificant appear- 

 ance, forming flat and generally very thin crusts; only rarely they may assume another appearance, 

 showing a massive exterior, but this is then due to the manner of growing, such species more or less 

 enclosing their substratum, which is in such cases loose bettom material, and getting thereby the 

 massive outer shape, but being in reality incrusting and with all the skeletal spicules fixed on 

 particles of the imbedded bottom material. The dermal skeleton may sometimes be strongly 

 developed, giving the sponge some thickness, and this is also the case just in the species appearing 

 massive, which also contributes to their massive appearance. The main skeleton is very uniformly 

 constructed, all the acanthostyles having their heads placed on the substratum and being more or 

 less vertical; only the density in which the spicules are placed may be somewhat different; generally 

 the acanthostyles are placed uniformly scattered over the substratum, but in some cases this is not 

 so, the acanthostyles being gathered in small bundles, from which then the fibres of the dermal 

 skeleton issue, and in the species, which enclose their substratum, this is generally so. The dermal 

 skeleton, which is less influenced by the incrusting growth, may be a little more varied, and is some- 

 times by far the most developed part of the sponge; in the thin, incrusting forms, however, it only 

 consists of more or less scattered bundles or short fibres. In the species in which the microscleres 

 are chelae, which is by far the greatest number, these may be more or less numerously scattered in 

 the dermal membrane, and not rarely they form a more or less dense, sometimes very dense, layer; 

 this feature reaches, I think, its climax in H. crux where the spined chelae form a very dense layer 

 constituting a protective mail. When the chelse are so strongly gathered in the dermis, I think this 

 is to some degree due to contraction, a state which generally prevails in spirit material and there- 

 fore is so often present in the material for examination; when the sponge is living and expanded I 

 think the chelse would be found much less concentrated; if this is so, it would also be easily 

 understood, that the chelae, when the living sponge is disturbed and contracts, form a strong, pro- 

 tective layer. 



The siirface is generally simple, more rarely it bears papillae, which may be very low, conical 

 warts, or somewhat long papillae, or they may finally be very long and thread-like; the papillae as a 

 rule bear the oscula and pores. I think that special adaptations for oscula and pores are also otherwise 

 often present, but they are often so insignificant, that they are difficult to detect, e. g. the pore-sieves 

 in H. Dujardinii, and when no special adaptations are seen I think the oscula are simple openings and 

 the pores lie in more or less pronounced groups over the subdermal cavities. 



The colours of the species are generally the ordinary yellow, greyish or brownish colours; 

 more rarely other, more lively, colours are present, as reddish, green or blue. 



As said above, some species grow on loose bottom material as sand, gravel, sponge-spicules 

 and the like; otherwise the numerous species form incrustations on every kind of substratum present 



