THE LITHISTII)^. 



329 



Fig. 15. SECTION THROUGH THE RIND OF A 



GEODINE SPONGE. (After Sollas.) 



The Moiiaxonidse are cosmopolitan, chiefly shallow-water forms. They range from the strand-line 

 down to 862 fathoms ; on the evidence of Cliona borings in Silurian fossils they are concluded 

 to have been in existence in early Pakeozoic times ; fossil remains of their skeletons are rare, 

 Pharetrospongia strahani (Sollas) of the Cambridge Greensand, a large Renierine Sponge with a 

 fibrous skeleton, being the best preserved and most certainly demonstrated example yet known. 



Tetractimllldce.* This sub-order embraces two very different groups; the Choristidce, in which 

 the spicules are separate from each other, and the Lithistidce, 

 in which they are united by the interlocking of their 

 branched ends into a dense stony network. 



In the Choristidwfare united a number of very different 

 types of Sponges, of which the Geodina are best known. 

 In these the body, usually more or less spherical, is differ- 

 enced into an external rind and an inner mark, or paren- 

 chyma, like an orange ; the rind (Fig. 15) consists of a layer 

 of fibrous connective tissue, covered externally by the ecto- 

 derm and a layer of minute flesh-spicules ; its outer two- 



tliiivlsi i fv-iiiiniprl full r>f sinifMilpts nsnallv trifliifp-cylobiilps t, Kelntimms ronm-ctivo tissue; e.excorrent canal, closed 



I6b, U dliy I te glOUUie by tho , uuscle (m) . ^ flt)rous tis | uo . ti j n c mTe ut canal. 



(mistaken by Bowerbank for ova), which give to it great 



firmness and consistency. The incurrent canals in their passage through the rind present a very 

 definite, usually hour-glass form, the constricted part being defended by a sphinctral muscle, produced 

 by a modification in the character of the surrounding fibrous layer. The chief spicules are large 

 acerates, which lie in bundles or fibres radiating towards the surface, near which some of them 

 divide into three rays, forming forks and anchors. 



The Litldstidce.\ Notwithstanding the firmness of their coral-like skeleton these are no more 

 characterised by constancy of form than the other groups of Sponges ; they usually affect cup-like, 

 lamellar, top-shaped, or cylindrical forms, are occasionally branched bush-like ; generally attached, 

 sometimes by a longer or shorter stalk, which branches out root-like below. Their skeleton consists of 

 the united body-spicules (Fig. 10, k) ; of surface spicules, anchor- or fork-shaped, or disciform ; and 

 minute so-called flesh-spicules. Differences in the character of the body-spicule have afforded Zittel a 

 means of dividing this group into four families, in one of which, the Megamorina, the quadri-radiate 

 form of the body-spicule is nearly lost, and the central canal or axial-thread has a simple uniaxial 

 form (Fig. 10, m). 



The Lithistidae occur in the Atlantic, Pacific, and Indian oceans ; they are essentially deep-sea 



dwellers, ranging from 74 to 805 fathoms. By reason of the com- 

 paratively large size of their body-spicules, and the union of these 

 into a stout resistent framework, they stand a much better chance of 

 fossilisation than the Monaxonidaa ; they are accordingly by no 

 means rare in a fossil state, and have been found in. most marine 

 formations from the Upper Cambrian to the Tertiary. The well- 

 kiiown Siphonia of the Blackdown Greensand is a familiar instance 

 (Fig. 9, D). 



Hexactinellidce. These Sponges are clearly defined from all 

 other Silicispoiigise by the sex-radiate form of their spicules, and by 

 the characters of their soft tissues so far as these are known. The 

 chief spicules are either loosely arranged into a fibrous skeleton 

 (Lyssakina, Fig. 17, A) or cemented into a solid network by a 



pores;/, flagellated chambers. deposit of silica (Dictyoiiina, Fig. 17, B), in which they are as 



completely enveloped as the spicules of Chalina in spongin. A 



slight deposit of silica may unite together the spicules of some Lyssakina, e.g., Euplectella, but 

 never to the extent of completely enveloping them. 



The Lyssakina^ include such forms as Holtenia, a somewhat cylindrical Sponge, with a large 



* Greek, tetra, contr. of tcttara, four ; actis, a ray. 



+ Greek, tithos, stone. 

 280 



t Greek, choris, separately. 

 Greek, lysis, loosing. 



