PROTOZOA. 



They consist of a pear-shaped mass, supported upon a longer 

 or shorter stem, which breaks up at its base into a number of 

 root-like processes of attachment. At the summit of the pyri- 

 form head is a single chimney-like osculum. In many respects 



Fig. 17. Siphonia 

 ficus, a Cretaceous 

 Sponge. D'Orbigny. 



Fig. i%.Ventriculites 

 radiatns. White Chalk. 

 (After Lyell.) 



the Siphonia present a curious resemblance to the Holtenia 

 of the Atlantic ooze, and, like these, they probably were 

 denizens of a deep sea. The genus, however, is not known 

 to occur in strata younger than the Chalk. 



Still more closely allied to the living Holtenia are the Ven- 

 triculites of the Chalk (fig. 18). These "have usually the 

 form of graceful vases, tubes, or funnels, variously ridged or 

 grooved, or otherwise ornamented on the surface, frequently 

 expanded above into a cup-like lip, and continued below into 

 a bundle of fibrous roots. The minute structure of these 

 bodies shows an extremely delicate tracery of fine tubes, some- 

 times empty, sometimes filled with loose calcareous matter 

 dyed with peroxide of iron " (VVyville Thomson). Like the 

 Siphonia, the genus Ventriculites is not known to occur above 

 the Chalk. 



The Tertiary Sponges call for no special comment ; but it 

 may be noticed that the great apparent predominance of the 



