104 SPONGES 



the more primitive forms the whole skeleton is made up of tri- 

 radiates alone. The primitive orientation of the triradiates, found in 

 the Olynthus, is only retained, as a rule, in the region of the oscular 

 tube, while in the tubar system generally the arrangement becomes 

 confused so that posterior and lateral rays cannot be distinguished by 

 their position. In some forms, however, characterised by a more erect 

 growth, such as CL bianco, and lacunosa (Fig. 8), the posterior ray is in- 

 dicated by its greater size, so that the triradiates become sagittal, while 

 remaining equiangular (Fig. 72, c). In lacunosa this feature is carried to 

 an extreme in the stalk, where a distinct peduncular skeleton is developed, 

 composed partly of sagittal triradiates (Fig. 72, d), partly of diactinal 

 monaxons, i.e. reduced triradiates (Fig. 72, h). Some species of Clathrina 

 have triradiates of special form on the exterior of the body, as an instance 

 of which may be mentioned the "tripods" of Cl. cerebrum (Fig. 72, g). 

 In forms with a distinct pseudoderm this membrane may be supported 

 by a layer of special spicules forming a dermal crust. 



In the Leucosoleniidae .the triradiate systems, if symmetrical, are 

 always sagittal that is to say, alate forms, with paired angles and 

 well-marked posterior and lateral rays (Fig. 72,^', &, I). Monaxons are 

 always present in the species of this family (Fig. 72, q, 8). The 

 sagittal form of the triradiates is correlated with the more erect growth 

 of these forms, and the spicules in question have a constant orientation 

 with regard to the canal system that is to say, they tend to be so placed 

 that the unpaired posterior ray points in the opposite direction to the 

 course of the water-current. Hence in the oscular tubes the posterior rays 

 point, as in the Olynthus, towards the base, while in the diverticula the 

 triradiates become arranged with their posterior rays pointing towards 

 the blind apex (Fig. 73), and the same arrangement is repeated in the 

 secondary and tertiary diverticula formed by branching, so long as they do 

 not exceed a certain length. In this way the diverticula, though arising 

 a# simple folds of the wall of the oscular tube or Olynthus, acquire a 



special skeleton of their own, distinct 

 osct from that, of the oscular tube in its 



arrangement, though not as regards 

 the spicules composing it. When the 

 diverticula have grown to a certain 

 length, however, they give rise to new 

 oscula which are formed by perforation 

 of their blind extremities. Where a 

 new osculum is about to be formed, 

 the arrangement of the triradiates 

 which are formed at the growing ex- 

 tremity of the diverticulum first be- 

 Flo- 78 ' somes confused, and then reversed, 



Diagram of a diverticulum of Leucosolenia. B _ f y. nf f u. torminni rfi f^a 

 showing the arrangement of the sagittal 8O tnat m tne t* rmi nai portion the 



triradiates in the oscular tube (otc.t.) and unpaired rays point awav from the 



in the diverticulum (div.). The arrow , -*, .. T xi-- 



points towards the oscular opening, apex instead of towards it In this 



way the arrangement proper to an 

 oscular tube is acquired precociously, at a time when the physiological 



