232 



PORIFERA 



of apparently identical larvae by both sexual and asexual 

 methods, has been observed in the Coelenterate Gonionema 

 murbachii} 



Artificially, sponges may be reproduced with great advantage 

 to commerce by means of cuttings. 

 Cuttings of the bath sponge are fit 

 to gather after a seven years' 

 growth. 



The development of the various 

 forms of spicules is a subject about 

 which little is yet known. Most 

 spicules of which the development 

 has been traced originate in a single 

 dermal cell. The triradiate and 

 quadriradiate spicules of Homocoela 

 (Clathrinidae), as Minchin '■^ has most 

 beautifully shown, form an excep- 

 tion. Three cells co-operate to form 

 the triradiate ; these three divide to 

 give six before the growth of the 

 spicule is complete. A quadriradiate 

 is formed from a triradiate spicule by 

 addition of the fourth ray, which, 

 again, has a separate origin in an 

 independent cell, in fact a porocyte. 

 The triradiate spicules of the Sycet- 



Fiu. lis. — Develojimeiit ol monaxoii . . . 



spicules. A, from Spongiiia tidac, on the other hand, originate 

 /«c«s<m, showmg the .single j^ a sinfde cell,^ but the quadri- 



scleroblast. (After Evans.) B, a 



qm 



very large monaxon, from Leuco- radiate spiculcs are formed from 



sdema, on which many sclero- ^| ^ ^j addition of a fourth 



blasts are at work. (Alter Maas.) J 



ray in a manner similar to that 

 which has just been described for Clathrinidae. 



Monaxon spicules if not of large size undergo their entire 

 development within a single scleroblast (Fig. 118, A). In some 

 cases if their dimensions exceed certain limits, several cells take 

 part in their completion ; some of these are derived from tlie 



' Perkins, Johns Hopkins Univ. Circ. xxi. 1902, \>. S7. 



- For details of this interesting process see Mincliin, Qiuirt. J. Micr. Sci. xl 

 1898, p. 469. 



" Maas, Zeifschr. wiss. Zool. Ixvii. 1900, p. 225. 



