DEVELOPMENT OF SPICULES 



division of tlie original scleroblast, otliers are drawn from the 

 surrounding tissue. In TctJiya, for example, and in Lcueosoleriuc ^ 

 the scleroblasts round the large monaxon spicules are so numerous 

 as to have an almost epithelioid arrangement. 



Tlie large oxeas of Tetilla, Stellctta, and Geodia, however, are 

 formed each within a sinude scleroblast." 



o a<> 



^{vJff'AA.A^ 



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

 spicules. (After Maas.) B («-e), of C7to«c?ri7te, from a spherical globule. (After Keller.) 



Triaenes have been shown ^ to originate as monaxons with 

 one swollen termination, froifi 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 microscleret 

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

 spherasters of Tethya are 

 formed by the union of 

 minute tetractine cal- 

 tlirops (Fig. 119,A). If 

 this view should be con- 

 firmed, it would afford a 

 very strong argument 

 for the Tetractinellid 

 afhnitios of Tethya. 



Keller,* on the otlier 

 luiud, tinds that the spherasters of the Tetractinellid Chondrilla 





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

 scleres oi Placospongia. (After Keller.) 



' Maas, SB. Ah. Milnchcn, xxx. 1900, p. 553, and Zeitschr. loiss. Zool. Ixx. 

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

 - Sollas, Challcnrjcr Monogrcqih, xxv. 1888, p. xlv. 

 = Sollas, ibid. pp. 13 and 34, pi. v. ^ Zeitschr. iviss. Zool. Hi. 1891, p. 294. 



