96 THE FAUNA OF THE DEEP SEA 



Pennatulid^e. The first three of these divisions 

 principally inhabit the shallow water. Each of them 

 sends a few representatives into the great depths, but 

 by far the greater number of the genera and species 

 are to be found between tide-marks or in depths of 

 less than fifty fathoms. 



The Pennatulids, on the other hand, are rarely 

 found in very shallow water, and nearly half the 

 known genera live in deep water. At least two 

 families may be said to be characteristically abysmal. 

 These are the Umbellulid^ and the Protoptilidge. 



The Pennatulid^e are regarded by naturalists as 

 the most complicated or highly organised group of 

 the Alcyonaria. Three different forms of polype build 

 up the colony or sea-pen as it is called. There is 

 a single very much modified and enormously large 

 polype, without tentacles, forming the axis, a large 

 number of ordinary Alcyonarian polypes (autozooids) 

 arranged in the form of leaves, or simply scattered 

 irregularly on the surface of the central polype, and 

 a number of very small undeveloped polypes 

 (Siphonozoids) without tentacles, whose function 

 seems to be to pump water into the canals of the 

 colony, and thus to keep up the circulation of water. 



The deep-sea genus Umhellula possesses a very 



