688 



ON CONCRETIONS, SPICULES, ETC. 



[CH. 



{S. inhaerens and S. digitata) are mainly distinguislied from one 

 another by the number of holes in the plate, that is to say, by the 

 average number of cells in the little cluster of which the plate or 

 spicule was formed. In many or perhaps most other holothurians 



Fig. 316. Development of anchor-plate in Synapta. After Semon. 



the spicules consist of little perforated plates or baskets, developed 

 in the same way, about cells or vesicles more or less close-packed, 

 and therefore more or less symmetrically arranged (Fig. 316). 



Fig. 317. Spicules of hexactinellid sponges. After F. E. Schultze. 



The six-rayed siliceous spicules of the hexactinellid sponges, while 

 they are perhaps the most regular and beautifully formed spicules 

 to be found within the entire group, have beert found very difficult 

 to explain, and Dreyer has confessed his complete inabihty to 

 account for their conformation*. But, though it may only be 



* Cf. Albr. Schwan, Ueber die Funktion des Hexactinellidenskelets, u. seine 

 Vergleichbarkeit mit dera Radiolarienskelet, Zool. Jb., Ahth. allg. Zool. u. Physiol. 

 XXXIII, pp. 603-616, 1913; cf. V. Hacker, Bericht uber d. Tripyleenausbeute 

 d. d. Tiefsee-Exped, Verh. d. zool. Ges. 1904. 



