n8 



THE INVERTEBRATA 



Class HEXACTINELLIDA 



Sponges with a purely siliceous skeleton composed of six-rayed 

 spicules; with small choanocytes and thimble-shaped flagellated 

 chambers ; and without jelly, the soft parts of the body being united 

 solely by a meshwork of trabeculae furnished by branching cells of 

 the dermal layer. 



A deep-sea group. 



Euplectella, Venus' flower basket, and Hyalonema, the glass-rope 

 sponge, have both been dredged in British waters. Both harbour 



pst. ih.ch 



ostihxh. 05^ 



•, / th.ch' / 



spi' 



Fig. 104. Section of a portion of Grantia extusarticulata. Highly magnified. 

 From Dendy. ost. openings of the inhalant canals (ostia) ; ih.ch. inhalant 

 canal; prp. openings of inhalant canals into flagellated chamber (prosopyles) ; 

 fl.c. flagellated or collar cells (choanocytes); ^.c/z. flagellated chamber; spi. 

 spicules; ap. exhalant opening (apopyle) of flagellated chamber. 



various commensal crustaceans. On the rooting- tuft of long, fine 

 spicules, which is the "glass-rope" of Hyalonema^ grows an epizoic 

 anemone of the genus Episoanthus. 



Class DEMOSPONGIAE 



Sponges whose skeleton, if present, does not contain six-rayed 

 spicules of silica, and may be purely siliceous, or composed of silica and 

 spongin, or of spongin alone; whose flagellated chambers have small 

 choanocytes and are usually small and rounded; and which possess 

 jelly. 



