INVERTEBRATE BOTTOM FAUNA 485 



digitattun, belonging to the same group, upon hard bottom in the 

 Httoral zone. We must also include among the alcyonaria the 

 sea-tree, Paragorgia ai'borea (see Fig. 343), which is taller than 

 a man and has many branches. Of true corals we may mention 

 Lophohelia prolifcra and AinpJiihelia raviea, though the coral 

 fauna is not regularly distributed over the hard bottom, but is 

 more or less local ; still 

 there are often numbers 

 of individuals where 

 hard bottom does occur. 

 Several species of hy- 

 droids, such as Lafoea 

 ditmosa, Sertularella 

 gayi, ' etc., are very 

 common ; and of the 

 bryozoans, Retepora 

 beaniana, easily recog- 

 nisable owing to its 

 trellis-like structure, is 

 both widely distributed 

 and plentiful. So, too, 

 are the brachiopods, 

 Terebratulina capiit- 

 serpentis and Wald- 

 heiuiia a^anmin, and 

 the two tube - worms, 

 Placostegus tridentahis, 

 the tube of which divides 

 into three tooth - like 

 processes, and Serpiila 

 verinicularis (see Fig. 

 344). Both these worms, 

 it may be added, have 

 calcareous tubes, in 

 contradistinction to the „ ^ , J^^' ^^^: , . 



Branch of Paragorgia arborea, L. 



tube-worms of the mud 



which inhabit tubes of mud or sand. There is, besides, a species 

 of barnacle ( Verruca strmni) on the stones, which is frequently 

 nearly as abundant as Balamis balanoides in the tidal area. 



It would take too long to give a full description of the 

 unattached fauna associated with the hard bottom. I will 

 therefore merely point out that some free forms occur only 

 upon the attached forms, and seem accordingly to be dependent 



