98 THE FAUNA OF THE DEEP SEA 



long and delicate axial polype, and the Autozooids and 

 Siphonozooids form a little cluster only at its extreme 

 summit. The small number of these polypes and 

 the very limited area over which they extend are the 

 two most characteristic features of the genus. It 

 would take me too far into the anatomy of the group 

 if I were to add any further details ; but I cannot pass 

 on without noting that the whole structure of 

 Umbellula shows that it is far more primitive and 

 simple than the shallow- water genera. And, generally 

 speaking, this holds good for all the deep-sea Pen- 

 natulids. In fact, we have here one of the rare ex- 

 amples of a series of genera, that can be regarded as 

 a slightly modified ancestry of the shallow-water 

 genera, that has been brought to light by the explora- 

 tion of the abysmal depths of the ocean. 



We have seen, then, that of the Coelentera, the 

 only order that has a large proportion of its genera 

 living in deep water, is the only one whose members 

 all possess a stalk by which they fix themselves into 

 the mud or sand at the bottom of the sea. 



It is not uninteresting to note, then, in passing 

 on to the Echinoderma, that the stalked Crinoids, the 

 only Echinoderms that can permanently fix themselves 

 to the bottom, are nearly all found in deep water. 



