84 



PORIFERA. 



2. Stylus, growth proceeds along the axis in one direction only hence 

 monactine : 



a. Style strongylote at origin (Fig. 72, 13]. 



b. Tyloslylctjiote at origin. 



Desmas (Fig. 72, 14, 15, 16) are megascleres which form the skeletal network 

 of the Lithistida. They are usually formed by the deposition of successive 

 layers of silica upon an ordinary spicule the crepis the axial rod of which 



is arrested in development. 

 When the crepis is a mo- 

 naxon we get a rhabdocrepid 

 dcsma, and when a tretraxon 

 the desma is called tetra- 

 crepid. 



II. Tetraxons are mega- 

 scleres with four actines 

 inclined at an angle, of 

 about 110 to one another. 

 Growth proceeds in one 

 direction only along each 

 of the four axes. There are 

 two kinds : (a) the calthrops, 

 when all four actines are 

 present ; and (b) the triad 

 (Fig. 71), when one actine 

 is suppressed, the remaining 

 three lying in one plane. 

 The triod is characteristic 

 of the calcareous sponges. 



III. The Triaxons (Fig. 

 78) are megascleres with six 

 actines, consisting typically 

 of a system of three equal 

 axes intersecting at right 

 angles. They are character- 

 istic of the Hcxactincllida. 



IV. Polyaxons are mega- 

 scleres with several axes 

 proceeding from a centre. 



V. Spheres are mega- 

 scleres in which growth is 

 concentric round a central 

 point. 



Yin. 73. Microscleres (from Perrier, after Sollas). 1, 

 globule ; 2, 3, sigina spires ; 4, 5, 7, sigmas ; G, micro- 

 triod ; S, transition between sigina and microtriod ; 9, 

 toxa ; 10, spirula ; 11, microstrongyle ; IS, spiraster ; 

 13, amphiaster ; 1U, metaster ; 7.", 1C, plesiaster ; 17, 

 chiaster ; IS, spiraster ; 10, samidaster ; 20, anthaster ; 

 21, microxea; S3, oxyaster ; 23, microxea ; 24, micro- 

 triod ; ~~i, orthodragma ; 26, monolophous microcal- 

 throps ; 27, dilophous mierocalthrops ; 2S, simple 

 microcalthrops ; 20, elongated sterraster; 30, trilo- 

 phous microcalthrops ; 31, tetralophous microcalthrops ; 

 32, spheraster ; 33, centrotylote ; 34, sterraster ; 35, 

 candelabrum ; 3d, pycnaster ; 37, microstrongyle. 



MICROSCLEKES. 



Microscleres (auxiliary spicules of Bowerbank, flesh spicules of Carter) are of 

 two chief kinds (Fig. 73). 



1. Spires are microscleres with a spiral twist. There are many varieties, of 

 which we may mention : 



a. Sigmaspirc a C- or S-shaped spicule according to the aspect (2, 3). 



b. Sigma a C-shaped spicule, not spirally twisted (5, 6, 7). 



c. Toxa a bow-shaped spicule, not spirally twisted (9). 



