542 



SPONGES. 



sponges ; the siliceous needles becoming less numerous and the horny substance 

 more abundant, till, in the true horny sponges, such as the toilet-sponges, the 



skeleton consists entirely 



of horny fibres. The 



ocellated Chalina, fre- 

 quently cast ashore round 



the British coasts, ex- 

 hibits an intermediate 



condition between the 



siliceous and horny forms. 



By teasing out and ex-. 



amining a few fibres under 



the microscope, a fine 



core of siliceous spicules 



will be seen in the axis of 



each thread of spongin. 

 The illustration on 



p. 528 represents a group 



of sponges growing to- 

 gether. Near the base of 



the black seaweed on the 



stone there arises the 



much -branched Desmaci- 



dine sponge, so called 



from its buckle - shaped 



spicules; while from the 



left branch of the former 



grows a flat alga encrusted 



by another sponge of the 



same order, and of a dull 



yellow colour ; and at the 



top of the colony is the 



violet Spongelia. The 



illustration to the left 



represents another sponge 



of this group (Axinella), 



common in the Mediter- 

 ranean. In life this 



sponge is of a yellowish 



colour, and the oscules present a radiate 

 arrangement like the polyps of a branch of 

 fan-coral. The illustration on p. 543 illustrates 

 a sponge dredged by the Challenger, and known 

 as Esperiopsis challengeri, from six hundred 

 and thirty fathoms, east of the Celebes. The largest specimens are about 8 inches 

 in height. From a solid, strong stem six or seven stalks are given off at gradually 



A SINGLE-RAYED SPONGE, 



Axinella (nat. size). 



SILICEOUS SPICULES OP MONAXONID SPONGES 



(magnified 200-300 diameters). 



