SPONGES. 



71 



as a supporting column of elongated spicules bound together and growing in a spiral 

 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 upper end compensate for this and form a 

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

 a perfect specimen. The stem in the living 

 sj)onge is always enveloped in the fleshy tissues. 



In Holtenia we have a different type of 

 sponge, similar in shape to the members of the 

 Calcarea, but the resemblance goes no further. 

 The star-like beauty of the external covering of 

 spicules, and the singular profusion of anchor- 

 ing threads which are formed below, are shown 

 in the adjacent figure. Dactylocalyx is another 

 of the open vase forms which occur in this sub- 

 order. 



The fossils are very numerous, and it is 

 supposed that several of the Cambrian sponges 

 may belong here, though Zittel cites only cer- 

 tain Silurian genera like Astylospongia and 

 Protospongia 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. 



Fig. 61. — Section of the outer wall of Ventriculites 

 simplex, sliowlng the structure of the silicious 

 network. 



Alpheus Hyatt. 



Fig. 62. — Spicule of Phermiema. 



