REPORT ON THE MONAXONIDA. 



XXVll 



ectosome, and (2) the Mcdn skeleton (PL XLVII. fig. 3, m.s.), which forms the main 

 mass of the skeleton, supporting the canal system, &c., in the interior of the sponge. 

 The dermal skeleton is very frequently distinguished from the main skeleton by a 

 different arrangement of the component parts, which is often very striking. It may be 

 either reticulate, composed of more or less definite fibres, or of scattered spicules laid 

 horizontally, or radiate, composed of spicules arranged vertically to the surface with their 

 apices projecting outwards. Thus in Plumohalichondria mammillata, the dermal 

 skeleton is a very compact and regular, polygonally-meshed reticulation of spiculo-fibre 

 (PI. XLVII. fig. 4), while the main skeleton is radiately arranged, consisting of ascending 

 columns of plumose fibres (PI. XLVII. fig. 4a, p.c.) running vertically to the surface; 

 this is an excellent example of a reticulate dermal skeleton combined with a radiate main 

 skeleton. In Myxilla rosacea, var. jaiDonica, on the other hand, we have a radiate 

 dermal skeleton combined with a reticulate main skeleton ^ide PL XLVII. fig. 3). 

 A radiate dermal skeleton and a radiate main skeleton, however, generally occur together. 

 Thus in the genera Suberites and Stylocordyla (woodcut, Fig. VIII.) the dermal skeleton 

 is composed of brushes of small, outwardly projecting spicules, often so closely packed 

 together as to form a dense, velvet-like pile. 



Very commonly, also, the individual spicules of the dermal skeleton differ from those 

 of the main skeleton. This difference may concern merely the size, as is the case in most 

 SuberitidaB, or it may concern the shape of the spicules. In Tedania, and again in 

 Myxilla, we always find a diactinal dermal spicule (oxeote, tornote or tylote) combined 

 with a monactinal main skeleton spicule (stylote), and in the 

 Spirastrellida3 (Spirastrella and Latruncidia) there is usually a 

 special dermal crust of quite peculiar spicules (spirulse and 

 discastra) which we here consider as belonging to the category 

 of microsclera. 



In two genera (both new) of deep-sea Desmacidonidse, we have 

 discovered very extraordinary special dermal spicules in addition 

 to the ordinary spicular complement of the group to which they 

 belong. In the one case, Axoniderma, the spiculation would be 

 that of the genus Cladorhiza, were it not for the existence of a 

 dense external armature composed of a thick layer of large amphi- 

 asters ; and in the other case, Meliiderma, we have a stipitate 



sponge agreeing in spiculation with the genus Chondrocladia, except for the presence of 

 a layer of quite peculiar spicules (woodcut. Fig. IX. ) which encrust the stem. In both 

 cases there can be little doubt that the extra spicules have been acquired in order to guard 

 against the attacks of enemies, a purpose for which they are admirably adapted. 



Not infrequently the true dermal skeleton is replaced, more or less entirely, by foreign 

 bodies, such as grains of sand, sponge-spicules, &c. These foreign bodies may be 



Fig. IX. — Meliiderma stipitatum. 

 Spicules encrusting the stem. 

 X 400. 



