I40 



SPONGES 



m 



m. 



B 



(Dictyoceratiua) — an evolution wliich has probabl)' taken place in 

 more than one family of Ilalichondrina. 



Wheu distinct skeletal fibres are present, they are built U|) of spicules 

 accoixling 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 tyjie the tibre is made up of 

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

 spicules 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- 

 ance. The spicules so placed 

 are said to " echinate " the fibre 

 Fig. 93, B). 



(3) The Ectyonine type of 

 fibre 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 siuiilar 

 to those occupying the axis 

 (Fig. 93, C). 



Arrangement of the Skeleton 

 at Large. — In the more typical 

 Halichondrina the skeletal 

 fibres have a reticulate arrange- 

 ment, in which primary fibres, runl^ing 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 Suberitidac 

 and many Clarulina, and to some extent in the Axinellulae, 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 1)e distinguished from a 

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

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



(y) Kn'titoM. — 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. ' 



Tiie spongin filtres of Keratosa consist typically of two j^ortions, 

 a softer and nu»re grainilar medullary substance, occupying the a.xis, 

 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 convenient!}- distinguished. 

 In the solid or homo^nieous fibres, tlie axial substance is very small 



Fk). i>3. 



Types of skeletal tihr<> in the Monnxuiiida. A, 

 Renierine or Cimltnine type ; /«, Axinellid type ; (.', 

 Ectyonine type. 



