POEIFERA (sponges). 



33 



in the trabeculae ; some support the dermal membrane, some Gallery X. 

 the gastral membrane, while others stretch across the body- 

 wall between the two membranes ; in Lyssacina yet other 

 spicules project from the dermal membrane. 



Modern Hexactinellids all live in the deep sea, fixed to 

 the bottom or moored in the ooze by long tufts of rooting 

 spicules. The best known is the beautiful Venus's Flower- 

 basket {Euplectella). 



Sub-Class I. — LYSSACmA. The spicules of the skeleton 

 either remain separate or are united at a late period of 

 growth in an irregular manner by siliceous masses or small 

 transverse rods (synapticulse). 



Oeder I. — Hexasterophora. In some of the spicules 

 in the middle layers of the body-wall the rays branch, 

 forming rosette-like bodies called hexasters. The chief 



Fig. 9, — Fossil sponge spicules : Silicispongiae, Hexactinellida. a-f are 

 skeletal spicules ; g, h are flesh spicules. Six-rayed spicules are shown 

 in a,b,e,f; in c one ray is suppressed. Axial canals are seen in 

 a, e and /, and a and / have lantern nodes, e and / illustrate the 

 union of spicules to form the square mesh of Dictyonina, e heing from 

 Sestrodictyon and / from Coeloptychiuni ; g is called a pinule, and h an 

 amphidisc. a is enlarged 66 diam. ; b, c, f, 40 diam. ; e, 47 diam. ; 

 g, 134 diam. ; h, 114 diam. (After Hinde.) 



families are Euplectellidse, Asconematidse, and Eossellidie. 

 Fossil representatives of the last two have been found in 

 Eocene rocks. 



Order II. — Amphidiscophora. There are no hexasters, 

 but some spicules in the limiting membranes are in the form 

 of rods with toothed disc-like expansions at their ends ; they 

 are called amphidiscs (Fig. 9 h). There is always an 

 anchoring root-tuft. The living forms belong to the family 



D 



