SPONGES.-HALLMANN. 20- 



convey irresistibly the impression that they have been e\oh'ed 

 from forms related to existing species of Echinoclathria 

 through the replacement in the fibres of principal by auxiliary 

 megascleres. 



The principal and accessory styli in Clathria likewise ex- 

 hibit differences in regard to their mode of distribution. The 

 principal styli, whilst invariably functioning as coring spicules 

 in the main fibres and usually also in the transverse, occur 

 also in many species as interstitial spicules, and in some as 

 supplementary echinating spicules along with the accessory 

 spicules. In the latter case it not infrequently happens that 

 the principal and accessory styli are connected by an unbroken 

 series of intermediate spicule-forms. The accessory styli, 

 which with rare exceptions are spined, are not as a rule ex- 

 clusively restricted to the office of echinating the fibres, but 

 also occur in limited numbers intrafibrallyi and interstitiallv. 

 The particular mode of occurrence of the several spicules 

 is, in all probability, characteristic for any given species 

 and should accordingly be taken account of in any descrip- 

 tion which aims at completeness. The microscleres of 

 Clathria are typically isochelae palmatae and toxa, but one or 

 both forms may be absent. It is a question wheher species in 

 which the chelae are other than of the palmate type should be 

 allowed in the genus. Whilst there can be little doubt thai 

 palmate cheke and arcuate chelae represent two distinct lines 

 of development, and are likely to serve as valuable aids for 

 the separation of genera, the fact that it is not possible in all 

 cases to say whether a particular chela is more strictly palmate 

 or more strictly arcuate, renders it inadvisable at present to 

 depend upon them solely for the generic separation of species ; 

 although eventually no doubt, other characters not hitherto 

 employed in classification will be found to provide a means of 

 deciding in doubtful cases. In connection with the genus 

 Clathria (minus JVilsonella), however, it is quite possible 

 that no species with intermediate or ambiguous chela? occur, 

 and accordingly a new genus might well be established for 

 those species which possess chela? arcuatae ; meanwhile, such 

 species must, I suppose, be allowed in the genus. Kirk- 

 patrick2 has described two species from South Africa, one 

 of which, C. mollis is scarcely capable of being retained in the 

 genus, since not only does it possess auxiliary tornota and 

 isochelae arcuata', but is also characterised by the absence oi 



1 The occurrence of acanthostyles iu considerable numbers in the 

 fibres is extremely well exemplified in Clathria Hartmeveri, Hentschel 



(1911^ 



Kirkpatrick— Marine Investigations in South Africa, ii.. pp. 248, 249. 



