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. 
JModern 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 {Euplcctdla). 
Sub-Class I. — lyssacina. 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 (synapticulae). 
Ordeb 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. 'J.— Fossil sponge spicules : Silicispongiae, Hexactinellida. a-f are 
skeletal spicules ; g, h are Hesh spicules. Six-rayed spicules are shown 
in (1,1), I’, f; in c one ray is suppressed. Axial canals are seen in 
(I, (• and /, and a and / have lantern nodes, e and f illustrate the 
union of spicules to form the square mesh of Dict 3 'oniiia, c being from 
Sestrodictgon and / from Coeloptychimn ; g is called a pinule, and h an 
amphidisc. a is enlarged GO diam. ; h, c, f, 40 diam. ; c, 47 diam. ; 
g, 1B4 diam. ; h, 114 diam. (After Hinde.) 
iamilies are Euplectellidie, Asconematidai, and liossellidfc. 
fossil representatives of the last two have been found in 
Eocene rocks. 
Oruek II. — Anaphidiscophora. Tliere are no hexasters, 
ljut some spicules in the limiting membranes are in the form 
of rods with toothed disc-like expansions at tlieir ends ; they 
are caUed amphidiscs (Fig. 9 h). There is always an 
anchoring root-tuft. The living forms belong to the family 
D 
