SPONGES.-HALLMANN. 



139 



degree of differentiation between principal and accessory spic- 

 ules appears to have been attained in genera like Raspailia and 

 Echinodictyum and in those in which the microscleres are 

 isochelse palmatae and toxa, though amongst the last-men- 

 tioned there are species {vide Clathria caelata, sp. ri.) in which 

 an unbroken transition of spicule forms occurs between the 

 small echinating spined "accessories" and the large (often 

 quite smooth) principal styli of the fibre-core. Finally, in a 

 third group of genera, comprising the "Dendoricincc" and 

 certain "Ectyonince" {e.g., Ophlitaspongia, EchinocJathria, 

 WihoneUa, Agelas, etc.) the basical spicules are unequivocally 

 of but a single kind. The logical conclusion with regard to 

 these is that either a differentiation of their basical spicules has 

 never occurred or that one or the other of the resultants of such 

 a differentiation has subsequently disappeared in the course of 

 evolution ; in most cases, the probability is that the absence of 

 a second kind is due to loss, inasmuch as the (basical) spicules 

 actually present, usually exhibit, in the matter of form and 

 function, features which are more characteristic either of prin- 

 cipal megascleres or of accessory. The further consideration 

 of the spicules renders it necessary to take into account other 

 features of the skeleton, and particularly the fibres. 



In the Myxillina- skeletal fibres have originated in apparently 

 two quite independent ways, viz., by the upgrowth of processes 

 from the basal lamina, and by the "ingrowth" of strands of 

 auxiliary spicules from the superficial layer. In many cases, 

 however, the fibres are the product of both modes of forma- 

 tion. Fibres which are wholly or partly of basal origin are, 

 with possible exceptions, more or less sponginous and traverse 

 the whole extent of the sponge ; those of purely superficial 

 origin are, at the most, scantily provided with spongin and 

 proceed from the surface (usually?) only for a short distance 

 (as, for example in ''Echinodictyum''' arenosum, " Plumohali- 

 choiulria" gravida and Eusifer fistidatits).'^ The former might 

 be distinguished as basifugaJ, the latter as hasipetal fibres. 



The mode of origination of basifugal fibres in the Myxillinae 

 is capable of being explained as follows : — The spongoblasts 

 (and probably also the "basical" scleroblasts) which primi- 

 tivelv — it mav be presumed — were uniformly distributed over 

 the surface of the basal lamina, became at particular points on 

 it more closely aggregated. The consequent more rapid de- 

 position of spongin at these points produced at each of them 

 a thickening of the lamina which gradually assumed the 

 form, sav, of a papilla. Scattered over the surface of this 

 papilla, just as over other portions of the surface of the 

 lamina, though perhaps more closely, were scleroblasts 



1 Dendy— Proc. Roy. Soc. Victoria, ix. (n.s.). 1897. 



