140 



SPONGES 



f 



(Dictyoceratina) an evolution which has probably taken place in 

 more than one family of Halichondrina. 



When distinct skeletal fibres are present, they are built up of spicules 

 according to one of three distinct patterns or types, which have been 

 named from the families or sub-families which they characterise. 



(1) In the Renierine or Chalinine type the fibre is made up of 

 spicules, all of which lie parallel to the direction of the fibre. The 

 spiculea may be arranged in a single series, end to end, or in more than 

 one such series (Fig. 92, A-C, and Fig. 93, A}. 



(2) In the Axinellid type each component spicule is inclined at a 

 variable, but usually acute, angle to the axis of the fibre, giving it a 



feathery or " plumose " appear- 



,1 1 v / \ ,i / ance. The spicules so placed 



I' v\ J7, \V 1^ are said to " echinate " the fibre 



I' "i H (Fig. 93, B). 



(3) The Ectyonine type of 

 fihre combines the peculiarities 

 of the other two types, since it 

 is made up of a core of parallel 

 spicules covered by a superficial 

 layer of echinating spicules, 

 which are very rarely similar 

 to those occupying the axis 

 (Fig. 93, C). 



Arrangement of the Skeleton 

 at Larye. In the more typical 

 Halichondrina the skeletal 

 fibres h>ve a reticulate arrange- 

 ment, in which primary fibres, running vertically towards the sur- 

 face of the sponge, can often be distinguished from secondary fibres 

 crossing them at right angles (Fig. 93, A, B, and C). In the Suberitidae 

 and many Clavulina, and to some extent in the Axinellidac, the fibres 

 have a more radiate arrangement, running from a centre or axis to the 

 surface without any crossing fibres. 



In most Monaxonida, whatever the general arrangement of the 

 skeleton may be, a dermal skeleton can usually be distinguished from a 

 main skeleton. In other respects, however, the skeleton shows very little 

 specialisation in different regions. A root tuft is never present. 



(y) Keratosa. In the horny sponges the skeleton consists of 

 fibres of spongin, which in one instance, Darwinella, are found 

 combined with isolated spicules of the same substance. 



The spongin nbres of Keratosa consist typically of two portions, 

 a softer and more granular medullary substance, occupying the axis, 

 surrounded by concentric coats or lamellae of true spongin, forming 

 the cortical substance. According to the proportions of these two 

 constituents, two types of fibres are conveniently distinguished. 

 In the solid or homogeneous fibres, the axial substance is very small 



FIG. U8. 



Types of skeletal tibre in the Moimxonida. 

 Renierine or Chalinine type ; II, Axinellid type : 

 Ectyouine type. 



