84 



PORIFERA MEGAMASTICTORA 



origin in one direction only, along four axes arranged as normals 

 to the faces of a regular tetrahedron. Forms produced by growth 



from an origin in one direction 

 along three axes lying in one 

 plane are classed with tetraxons.* 



III. Triaxons. — Spicules in 

 which growth is directed from 

 an origin in both directions 

 along tliree rectangular axes. 

 One or more actines or one or 

 two axes may be suppressed. 



IV. Polyaxons. — Spicules in 

 which radiate growth from 

 a centre proceeds in several 

 directions. 



Y. Spheres. — Spicules in 

 which growth is concentric 

 about the origin. 



A distinction more funda- 

 mental than that of form is afforded by the chemical composition : 

 all sponges having spicules composed of calcium carbonate Ijelong 

 to a single class, Calcarea, which stands alone in the branch 

 Meo-amastictora. 



Fig. 76.— Types of megascleres. a, Rhalxliis 

 (monaxon cliactine) ; h, stylus (moiiaxoii 

 monactine) ; c, triod (tetrax'ou triactine) ; 

 d, calthrop (tetraxoii tetractine) ; e, 

 triaxon hexactine ; f, euaster. 



BRANCH I. MEGAMASTICTOEA 



CLASS CALCAEEA 



Calcarea are marine shallow- water forms attached for tlie most 

 XJart directly by the basal part of the body or occasionally by the 

 intervention of a stalk formed of dermal tissue. They are almost 

 all white or pale grey brown in colour. Their spicules are 

 either monaxon or tetraxon or both. The tetraxons are either 

 quadriradiate and then called " calthrops," or triradiate when 

 the fourth actine is absent. The triradiates always lie more or less 

 tangentially in the body-wall ; similarly three rays of a calthrop 

 are tangentially placed, the fourth lying across the thickness of 

 the wall. It is convenient to include the triradiate and the 

 three tangentially placed rays of a calthrop under the common 



