102 



SPONGES 



should be carefully distinguished. In the first place, the angles 

 between the rays may be equal, and the bilateral form is the result 

 of hypertrophy or diminution of one ray (Fig. 72, c, d). In the 

 second place, the angles may vary as well as the rays, there being 

 two lateral paired angles and an anterior unpaired one (Fig. 72, 

 y, /, n, o). In a natural classification of the triradiate systems, the 

 equiangular sagittal spicules should be classed with the regular 

 forms, and separated from those which are sagittal through varia- 

 tions in the angles. For the latter type Bidder has proposed the 



FIG. 72. 



Spicules of calcareous sponges. To the left (o-i) spicules of Clathrinidae ; to the ri^ht (j-s) 

 of Leucosoleniidae and Heterocoela. a and b, triradiates of Clathrina cerebrum, in profile view 

 and surface view respectively ; c, sagittal triradiate of Cl. bianco,; d, of Cl. lacunoM ; e, f, quadri- 

 radiates of Cl. cerebrum, with spiny gastral rays ; ff, " tripod " of Cl. cerebrum; h, diactine of 

 Cl. lacunosa ; i, monaxon of Ascandra falcata ; j, triradiate, and k, quadriradiate, of Leucosolenia 

 variabilis; I, triradiate of Lelapia australis; m, quadriradiate of Leucosolenia complicate; n, tri- 

 radiate of teucetta pandora ; o, "tuning fork" of Lclfijtia australis; p, asymmetrical triradiate 

 of Leucosolenia variabilis; q, monaxon of the sanio ; r and s, two kinds of monaxons, one small 

 and straight, one large and curved, from Leucosolenia iwnplicata. 



useful term alate spicules, since their rays can usually be distinguished 

 by their form as well as by their inclination ; the posterior ray being 

 as a rule straight, the lateral rays more or less curved, like wings 

 on each side. 



(c) Any of the numerous form varieties of the triradiate system, 

 symmetrical or asymmetrical, regular or sagittal, may become pro- 

 vided with an adventitious gastral ray, and so become a quadri- 

 radiate spicule. The gastral rays vary greatly in length, and may 

 be smooth or beset with small spines (Fig. 72, e, /, k, m). They 

 may further be straight or curved, the former being usually associ- 



