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



Polylophus 2^hiUppinensis (PI. LIV. fig. 2), and also the dermal diacts of Bathydorus 

 haculifer (PI. LIX. fig. 11). A ray projecting towards the exterior is found in the 

 pentact dermal pinuli of all Hyalouematidte and many Asconematidse, while a ray 

 extendino- inwards occurs in the dermal peutacts of Rossella antarctica (PI. LV. 

 figs. 2, 3) and other Eossellidae. Autodermalia with rays projecting both inwards and 

 outwards from the dermal membrane occur in Balanites (PL XXIII. figs. 13, 14), 

 Aulascus (PL XXII. figs. 2, 3), and as amphidiscs in all Hyalonematidse. 



As hypodermalia I would note, in the first place, those sword-like hexacts in the 

 Euplectellidge, which, with their short distal rays, raise the dermal membrane into small 

 peaks, and are usually surmounted by an attached floricome. Hypodermalia are 

 well illustrated, too, by those strong pentacts which, in many Hexactinellida, lie with 

 their four tangential rays closely under the dermal membrane, and bear a greatly 

 prolonged proximal, penetrating like a strong peg at right angles to the surface, more or 

 less deeply into the subjacent parenchyma. The tendency of these hypodermal pentacts 

 to grow inwards may be recognised where the axis cross occurs in the dermal membrane, 

 from the fact that the tangential rays frequently extend obhquely inwards from their 

 points of intersection, and always lie beneath any other dermalia which may be present 

 (PL XXXVI. fig. 1). 



To the dermal skeleton I refer finally those spicules which I have noted as clavulse 

 and scopulse. These forms are disposed at right angles to the outer surface, have the 

 greater part of their elongated uniaxial body embedded in the parenchyma, usually, 

 however, reaching the skin or even extending beyond it with their broadened terminal 

 portion, which contains the axial cross of the central canal. Here, too, we include those 

 fine raphides which sometimes occur in bundles close to the radial rays of other dermalia 

 (PL XCII. fig. 2 ; PL XCIII. fig. 2). 



Where the skin is raised above the rest of the parenchyma as an independent plate, 

 the spicules of the dermal skeleton are either confined to this plate without being con- 

 tinued on the outer side of the subjacent parenchyma, e.g., in Semjoerella schuUzei 

 (PL LII. fig. 3), and in Hexactinella lata (PL XCV. figs. 1, 2), or they occur not only 

 on the independent skin plate, but also on the outer surface of the parenchyma, which 

 then exhibits a special layer of skin, as in Euryplegma auriculare (PL CII. fig. 3), and 

 Myliusia zittelii. 



Gastralia. 



Eelations similar to the above are exhibited by the gastral skeleton which supports 

 the gastral surface and inner side of many efi'erent canals, and which, in some cases, 

 appears at the oscular border as a direct, and but slightly altered continuation of the 

 dermal (PL XXIII. fig. 14 ; PL LXVIII. fig. 1). The diff"erence between gastralia and 

 dermalia is, however, usually distinct enough (PL XXV. fig. 3), and the boundary 



