REPORT ON THE MONAXONIDA. XXV 



the outwardly projecting spicules. This arrangement prevails amongst the AxinellidEe, of 

 which grou}} it is very characteristic. 



3. Tlie Ectyonine type (PI. XLVI. fig. 9 ; PL XLVII. figs. 4a, 5).— This agrees with 

 the Renierine type, except that the surface of the fibre, instead of being smooth, is 

 " echinated " by spicules projecting outwardly and usually more or less at right angles. 

 The central axis of longitudinally placed spicules is generally present, but may, in a few 

 instances, disappear, leaving only the horny fibre and the " echinating " spicules {e.g., in 

 Clathria lendenfeldi, PI. XLVII. fig. 5). The echinating spicules are generally diS'erent 

 from those in the centre of the fibre, but may, though very rarely, be the same {e.g., 

 Efhinoclathria favus). Such fibre is characteristic of the subfamily Ectyoninae, and is 

 apparently confined to this group. 



These three principal types of fibre all occur in the Halichondrina. In all three the 

 amount of spongin present is extremely variable. In the Clavulina on the other hand, 

 w^here, it will be remembered, there is no spongin present, we are acquainted only with the 

 Renierine type of arrangement ; the fibres, when distinct fibres are present, being com- 

 posed of longitudinally placed spicules lying parallel with one another. 



In aU Monaxonid sponges there are usually a great number of megasclera scattered 

 irregularly through the tissues, in addition to those which enter into the composition of 

 the fibre. 



(2) The Arrangement of the Skeleton at large. 



In this case we have only two main types of arrangement to deal with. First, Reticu- 

 late, and second, Radiate. 



1. The Reticidate arrangement, characteristic of nearly aU the Halichondrina. In 

 cases where this arrangement obtains the spicules may either be arranged (at any rate 

 partly) in distinct fibres, or scattered separately through the soft tissues without any 

 arrangement whatever (as in Halichondria solida, nobis). The fibres, when present, 

 may belong to any of the three types above described. The essential character of a 

 reticulate skeleton is that the fibres, or individual spicules in cases where no fibres exist, 

 should cross and recross one another, so as to give rise to a network with closed meshes 

 (PL XLVI. figs. 2, 3, 4, 6, 9). AU gradations exist between a reticulation of separate 

 and quite irregularly scattered spicules and a reticulate skeleton with perfectly definite 

 rectangular meshes whose boundaries are formed of compact fibres (PL XLVI. figs. 3, 6). 



When the skeleton is rectangularly arranged (as in Renicra, Chalina, Esperella, &c.), 

 it is easy to distinguish between two sets of fibres — (1) Primary fibres (PL XLVI. 

 fig. 3, a), running vertically towards the surface of the sponge, and (2) Secondary fibres 

 (PL XLVI. fig. 3, V), crossing the primary fibres at right angles. When the primary fibres 

 are composed of longitudinally arranged monactinal spicules these have, as a general rule, 

 their apices all pointing in one direction, towards the surface of the sponge. At the 



(zooL. CHALL. EXP. — PART Lix. — 1887.) Nnn d 



