PRINCIPAL GROUPS OF THE SPONGES. 



I6 5 



ends or expanded articular surfaces. The spicules fonn an open mesh- 

 work by the apposition of their terminal facets. Typical genera are 

 Doryder??ia, Pachypoterion, and Carterclla. 



3. Anomocladi?ia. — In this group the skeleton-spicules have the form 

 of a central node from which radiate simple or bifurcate arms, which 

 have slightly expanded ends, and are variable in number. The skeleton 

 forms a regular network produced by the union of the expanded ends of 

 the spicular arms with the nodes, or sometimes with the branches of 

 adjoining spicules (fig. 55, a). Typical genera are Vetulma, Hindia, 

 and Astylospongia. 



4. Tttracladina. — In this group, the spicules (fig. 54) consist of four 

 rays which meet in a non-inflated centre in such a way that one ray 



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Fig. 54.— a, A single skeleton-spicule of the living Lithistid Discodermia polyciiscus, magni- 

 fied 60 diameters ; b, Small portion of the skeleton of Sithonia Ayriformiss similarly enlarged. 

 (After Sollas.) 



forms a shaft from which the other three rays spring, approximately 

 meeting at angles of 120 . The ends of the rays are branched, and 

 interlock with the ends of adjoining spicules to form a continuous mesh- 

 work. Typical examples are the genera Jerea (fig. 49, e), Aitlocopiitm, 

 and Siphonia (fig. 58). 



The Lithistid Sponges are all inhabitants of the sea, and the 

 existing genera (Discodermia, Corallistes, Vetuli?ia, &c.) are mostly 



