SPONGES. 



71 



in a sjilral 





as a supporting column of elongated spicules bound together and growin 

 as the animal progresses upwards. 



The lower end of the stem becomes frayed out, and sinks into the mud as the ani- 

 mal grows, but constant additions to the ujiper end compensate for this and form a 

 column which sometimes reaches a foot in length. In Fig. 59 we see on the ri"ht 

 a jierfect specimen. The stem in the living 

 sjioiige is always enveloped in the fleshy tissues. 



In llohenia we have a different type of 

 sponge, similar in shape to the members of the 

 Calcarcn, but the resemblance goes no further. 

 The star-like beauty of the external covering of 

 sjiicules, and the singular i^rofusion of anchor- 

 ing threads which are formed below, are shown 

 in the adjacent figure. Dactylocuhjx is another 

 of the ojjen vase forms which occur in this sub- 

 order. 



The fossils are \-ery numerous, and it is 

 supposed that several of the Cambrian sponges 

 may belong here, though Zittel cites only cer- 

 tain Silurian genera like Astijlospongia and 

 ProtosiyoiKjia as undoubted Hexactinellids. 



One of the best known of the fossil types is Ventriculites, our figures of which show, 

 not only the general shape, but the structure of the skeleton as well. 



'hnMriMi 



Fig. Cl. — Section of the outer waU of ]-entrieu1ltes 

 simpfex, sliowiug the structure of the silicious 



Alpheu.s Hyatt. 



Fig. 62. — Spicule of Plier 



