126 THE INVERTEBRATA 



it is best that, in a classification of animals, they should be given, 

 under the name of Parazoa, the same rank as the Protozoa and the 

 Metazoa. 



Class CALCAREA 



Sponges with skeletons consisting solely of calcareous spicules ; and 

 with large choanocytes. 



Clathrina. A meshwork of Ascon tubes. The nuclei of the choano- 

 cytes are at the bases of the cells. British. 



Leucosolenia. A clump of erect Ascon tubes, each of which may be 

 branched, connected at their bases. The nuclei of the choanocytes 

 are apical. British. 



Sycon (Fig. loo). A simple vase with a canal system of the second 

 type, having the thimble-shaped outgrowths little adherent to one 

 another. The nuclei of the choanocytes are apical. British. 



Grantia. Differs from Sycon in that the outgrowths which contain 

 the flagellated chambers adhere in many places and are covered by a 

 cortex (Fig. 105). British. 



Leucandra. Canal system of the third type (Fig. loi). Nuclei of 

 choanocytes basal. British. 



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 

 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 Epizoanthus. 



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. 



