PORIFERA 



125 



rays may grow out at their ends or elsewhere into spikes, knobs or other 

 projections as e.g. the middle spicule of Fig. 58, B. 



Well-known members of the group Hexactinellida are illustrated by 

 Figs. 59 and 60. 



Fig. 59 represents the " Glass Rope Sponge," 

 characterized especially by the tuft of gigantic 

 spicules, looking like a piece of rope of spun 

 glass, by which the sponge is rooted in the 

 mud (Fig. 59, r.s). Each of these rooting 

 spicules possesses at its free end an anchor-like 

 arrangement of recurved hooks which some- 

 times represent four reduced rays while at other 

 times they appear to be secondary outgrowths. 

 An interesting feature commonly seen in speci- 

 mens of Hyalonema is the presence of small 

 symbiotic anemones, attached to the portion 

 of the root tuft which was not buried in the 

 mud (Fig. 59, an). 



Fig. 60 illustrates the " Venus's Flower 

 Basket " sponge — Euplectella — often brought as 

 a curiosity from the East. Here the body of the 

 sponge forms a cylindrical tube, closed at the 

 top by a sieve-plate. The wall of the sponge 

 is supported by a beautiful trellis-work formed 

 of fused spicules, the strands of the trellis- 

 work — longitudinal, circular, right-handed, and 

 left-handed spirals — being so arranged as to 

 meet the stresses to which the wall of the 

 sponge is subject. When the sponge develops 

 in situations where there is a prevalent current 

 pressure in one direction the cylinder takes on 

 a correlated curvature as in the specimen 

 figured, the concavity of the curve facing the 

 current. Further strengthening of the sponge 

 may be brought about by the development 

 of spiral flanges projecting from the surface as in the specimen figured 

 (Fig. 60). 



III. Demospongiae 



Fig. 59- 



Skeleton of Hyalonema. 

 an Small anemones {Palylhoa} 

 growing on portion of root 

 above the surface of the mud ; 

 r.s, root spicules. 



The Demospongiae comprise a great variety of sponges including 

 most of the common and more conspicuous genera. The canal system 



