THE BOWERBANKJA DENSA. 99 



the same structure and the same connexion with the 

 cells as the flustrce, Dr. Grant has observed in living 

 specimens a constant motion of flexion and exten- 

 sion in certain remarkable testaceous processes, 

 shaped like a bird's head, and attached by a kind of 

 stem to the outside of all the cells. These processes 

 and organs are likewise provided with lateral folds, 

 like the valves of a shell, which have a distinct and 

 regular motion corresponding with the flexion and 

 extension of the entire process. 



One genus* comprehends a very singular group 

 of moss corals, of which the cells, in dense clusters, 

 are irregularly scattered on the stem, which is usu- 

 ally attached to the common seaweed. f The best 

 known species is the Sowerbankia densa of Dr. 

 Farre. In its early stage of development, the cells 

 do not spring from a stem, or raised polypary, but 

 spread in a single layer over the surface of the 

 seaweed ; and afterwards, a stem rises. Mr. Hassell 

 gives a figure of the perfect zoophyte, from which 

 the annexed figure is taken. 



It is, however, to Dr. Farre that we owe our 

 knowledge of the structure of this singular animal; 

 his account, which refers to the immature of the 

 species, we shall give as briefly as possible. 



The polyp inhabits a transparent horny tube, 



about a line in length ; and of these tubes, small 



clusters, arising from a common base, may not un- 



frequently be detected. When protruded from the 



* Valkeria. f Fucus vesiculosus. 



