PORIFERA 103 



forms, as the character of sediment and associated life forms 

 indicate, lived mostly in comparatively shallow water. They 

 are known as fossils, at least from the Cambrian to the present. 

 Euplectella aspergillum (Venus' flower basket) (Fig. 37, B). 



Living. 

 A tubular cornucopia-shaped sponge with a transverse, ter- 

 minal sieve-like cap at its summit and an anchoring root-tuft 

 at its base. The skeleton consists of siliceous threads, each of 

 which is probably the greatly elongated ray of one axis of a 

 three- to six-rayed spicule (the other rays being suppressed), 

 cemented in adult life end to end to others by the addition of 

 secondary silica.- This process results in the formation of long, 

 vertical, spiral and circular threads which form a polygonal net- 

 work around the central cavity. There is usually one ostium 

 in each quadrangular interval. The vertical threads terminate 

 in the conical root-tuft which forms the base. Upon the out- 

 side there are ornamental spiral ridges (whence probably the 

 name from Greek eu, well, + plektos, twisted). It grows at 

 considerable depths in the ocean fastened by the penetration of 

 the stiff threads of the root-tuft into the mud or ooze. This 

 species was dredged by the Challenger Expedition from a depth 

 of 95 fathoms, and E. crassistellata from a depth of 2750 fathoms. 



1. Sketch side view of entire specimen, labeling cap, root- 

 tuft, ostia. 



2. What is the composition of the skeleton? function of 

 the root-tuft ? 



3. Give the probable origin of the long threads. 



4. Describe the process by which Venus' flower basket could 

 leave a record of itself in the inclosing sediments. 



Prismodictya (Fig. 37, A). Devonian- Mis sis sip pian. 



An elongate, slender, gradually expanding, eight-sided prism ; 

 each prism-face reticulated by the spicular threads, which are 

 arranged regularly in larger and smaller quadrate meshes sit- 

 uated one wdthin another. Found as internal molds (16). 



This sponge was very similar to Venus' flower basket, though 



