48 THE VOYAGE OF H.M.S. CHALLENGER. 



Regular hexact. — A hexact with six rays at right angles to one another, of equal length 



and of similar form. 

 Oxyhexact. — Hexact Viiih rays running out to a point. 

 Sphaerohexact. — Hexact with spherical terminal knobs. 

 Discohexact. — Hexact with a transverse disc at the end of each ray. 

 Rosette or hexaster. — A hexact with equal terminal rays. 

 Oxyhexaster. — A rosette with straight or bent terminal rays, running out to a 



point. 

 Graphiohexaster. — A rosette where the ends of the principal rays bear a bundle of long 



fine terminal rays in a brush-like manner. 

 Sphierohexaster. — A rosette with spherical knobs at the ends of the terminal rays. 

 Discohexaster. — A rosette with transversely disposed, radially symmetrical discs on the 



outer ends of the terminal rays. 

 Floricome. — A rosette with S-shaped terminal rays, which are arranged in a whorl 



like the petals of a lily, and which pass at their outer end into a thickened 



terminal plate, arched outwards, and provided with external marginal teeth or 



claws. 

 Plumicome. — A rosette with numerous S-shaped terminal rays, whose external curved 



ends form several tiers, one upon another. 

 Pinulus. — A pentact or hexact in which one ray bears oblic^ue lateral teeth or prickles, 



presenting a resemblance to a fir tree or fir cone. 

 Scapula. — A fork-like spicule with a long straight stalk passing at one end into two or 



more teeth. 

 Amphidisc. — A diact, at each end of which a convex expansion occurs, which bears six 



or more backwardly bent marginal teeth. 

 Ancora. — A rod-like spicule at one end of which two, or several whorled, transverse 



spikes occur, which are bent backwards like hooks. 

 Uneinatum. — A straight rod, pointed at both ends, and beset all over with barbs point- 

 ing in the same direction. 

 Clavula. — A rod which bears at one end a club-shaped or transverse discoidal exj^ansion. 

 Prostalia. — All the larger spicules which project far beyond the outer surface of the 



body, and which may be readily seen with the naked eye. 

 Basalia. — The prostalia which project downwards from the lower end of the body, and 



which form the basal root-tuft by which the Sponge is fixed in the mud. 

 Pleuralia. — The prostalia which project beyond the outer surface of the lateral portion 



of the body. 

 Marginalia.— The prostalia which project in wreath-Hke arrangement round the oscular 



margin. 

 Dermalia. — All the spicules which stand in a definite relation to the external skin. 



