XXVI 



THE VOYAGE OF H.M.S. CHALLENGER. 



surface of the sponge, also, the spicules (whether monactinal or diactinal) composing the 

 primary fil)re very commonly spread out so as to form divergent brushes, upon which the 

 dermal membrane rests (PI. XLVII. fig. 6). 



In another important, though less common type the skeleton is regularly reticulate 

 without its being possible to distinguish between primary and secondary fibres. In this 

 case the meshes of the reticulation are triangular, with equal sides each of exactly one 

 spicule's length. Hence there can be no long, continuous fibres such as exist in a 

 rectangularly meshed skeleton, the greatest continuous length of fibre possible being 

 determined by the length of the spicules. Such a skeleton is found in Myxilla roscicea, 

 var. jcqMnica, nobis (vide PI. XLVII. fig. 3). 



2. The Radiate arrangement. — This is characteristic of the Suberitidae, and, to a less 



Fig. VIII. — Styloconlyla stipitata, var. ylobosa. Sections sho%ving the arrangement of tlie skeleton. A, longitudinal section 

 of a specimen ■which exhibits no spiral arrangement of the fibres, x 6. A', transverse section of do. , x 4. B, transverse 

 section of a specimen in which the fibres are spirally disposed, x 4 ; o, osculum ; c, cortical spicules ; a, stem ; cm, 

 central mass of spicules derived from the breaking up of the stem ; s, sections of do. ; rf, the radiating skeletal fibres. 



extent, of the Axinellidae. The fibres or bands of spicules radiate to the surface from 

 a common centre, and there are no crossing fibres {vide woodcut. Fig. VIII.). In the 

 Axinellidse the common centre of radiation is usually much elongated, so as to give rise 

 to a central axis of fibre from which other fibres radiate to the surface. The same may 

 take place in the genus Subeintes (e.g., Suberites elongatus, nobis, Suherites axiatus, 

 nobis, &c.). Thus the radiate type of skeleton difiers principally from the reticulate type 

 with regular rectangular meshes in that there are primary but no secondary fibres present. 



The skeleton of most ]\Ionaxonid sponges, whether reticulate or radiate in its arrange- 

 ment, is generally divisible into tw^o parts — (l) the Dermal skeleton (PI. XLVII. 

 fig. 3, d.s.), at the surface of the sponge, specially modified to support and protect the 



