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



octahedral nodes. Joannella, Schmidt, is distinguished by its narrow meshed network 

 and the marked fissures associated with this character. In one species of the new genus 

 Scleroplegma [Scleroiolegma laterna) there are perforated octahedral nodes, while in the 

 other species of the same genus the nodes are solid. In Volvulina sigsbei, 0. Schmidt, 

 which is distinguished by the knotted cords of the siliceous network of beams, Schmidt 

 thinks that he can prove the non-fixity of certain characters of the Dictyonine lattice-like 

 tissue, characters which have been principall)^ used by Zittel in the classification and 

 determination of fossil Hexactinellida. While, namely, in some parts of the lattice 

 network smooth rays and rough knots appear, in other regions of the same specimen the 

 rays are also rough. Many specimens of the same species exhibit rough rays and smooth 

 knots, the exact reverse of what has just been stated above. Schmidt has also found 

 that the form of the meshes is very variable and uncertain, they are sometimes cubical, 

 sometimes predominantly polyhedral, sometimes quite irregular. 



Of Euplectellidge Schmidt mentions, besides Eiqylectella jovis, which is a species 

 nearly related to JEiq^lectella suberea, Wyv. Thomson, a new genus Regadrella, including 

 the single species Regadrella phoenix, which is devoid of the siliceous tuft and is 

 fixed to the rocky substratum by a firm base. Though in this form a very constant 

 character of the Lyssacina is evidently absent, it must be noted that other forms occur 

 which are quite firm and compact beneath, lattice-like in the middle, and loosely 

 constituted above, such as, for example, Hertwigia falciformis, Schmidt, and Rhahdo- 

 pectella tintinnus, Schmidt, and these were direct transitional forms between Dictyonina 

 and Lyssacina. 



Of the genus Hyalonema Schmidt has described only fragmentary pieces. The form 

 described by him as Asconema hentii is certainly a species of Hyalonema. 



The then little known structure of the soft body of the Hexactinellida was, in 1880, 

 elucidated by my ^ investigation of some well-preserved specimens of Eiiplectella aspei'- 

 gillum, which were given to me by Sir Wyville Thomson from the treasures of the 

 Challenger expedition. 



In a subsequent study of the Hexactinellid material from the Gulf of Mexico, which 

 had been already utilised by 0. Schmidt, but which was re-examined by Weltner^ in 

 1882, the structure of the peculiar skin and covering layers was especially considered. 

 This had, it is true, been previously described by Zittel in several fossil forms, but in 

 living forms it had hitherto been insufiiciently known, and had been investigated, in 

 fact, only in a few cases, such as in Aphrocallistes by Zittel, in Myliusia by Marshall, and 

 in Dactylocalyx pumiceus by SoUas. Weltner was able to demonstrate the presence of 

 these structures in numerous modern Dictyonina, e.g., in Farrea, Syringidium, Aphro- 

 callistes, Volimlina, Joannella, Margaritella, Scleroplegma, and Cystispongia. Like 

 Zittel he distinguished " covering layers, dependent and independent of the lattice- 



1 Trans. Roy. Soc. Edin., vol. xxix., 2, p. 661. " Weltner, Beitriige zur Kenntniss des Spongien, 1882. 



