DEVELOPMENT OF SPICULES 



233 



division of the original scleroblast, others are drawn from the 

 surrounding tissue. In Tcthya, for example, and in Leucosolenia ^ 

 the scleroblasts round the large monaxon spicules are so numerous 

 as to have an almost epithelioid arrangement. 



The large oxeas of Tetilla, Stelletta, and Geodia, however, are 

 formed each within a single scleroblast.^ 



77' vr. 



>.^ 



3 ^ '' *■ 



dr 





A 



Fig. 119. — Development of spheraster. A, of Tethya, from union of two quadriradiate 

 spicules. (After Maas.) B (a-e), of CTj<nirfrj7Za, from a spherical globule. (After Keller.) 



Triaenes have been shown ^ to originate as monaxons with 

 one swollen termination, from which later the cladi grow out. 

 Information as to the scleroblasts in this case is needed. 



The value of a knowledge of the ontogeny of microscleres 

 might be great. Maas believes that he has shown that the 

 spherasters of Tethya are 

 formed by the union of 

 minute tetractine cal- 

 throps (Fig. 119,A). If 

 this view should be con- 

 firmed, it would afford a 

 very strong argument 

 for the Tetractinellid 

 affinities of Tethya. 



Keller,* on the other 

 hand, finds that the spherasters of the Tetractinellid Ghondrilla 



Fig. 120. — Stages in the development of the micro- 

 scleres of Placospongia. (After Keller.) 



1 Maas, SB. Ah. Miinchen, xxx. 1900, p. 553, and Zeitsohr. wiss. Zool. Ixx. 

 1901, p. 265 ; see also SoUas, Ann. Mag. Nat. Hist. (5) ix. 1880, p. 401. 



2 Sollas, Challmger Monograph, xxv.1888, p. xlv. 



s Sollas, ibid. pp. 13 and 34, pi. v. ■• Zeitsohr. wiss. Zool. lii. 1891, p. 294. 



