HYDEOID ZOOPHYTES. 35 



compartments by radiated partitions, which have reproduc- 

 tive functions; germs ejected through the orifice of the 

 cavity, — " Actinoidea." 



It is with the " Hydroicla'^ that we shall have to do prin- 

 cipally in this Chapter ; and instead of going more deeply 

 into their general description, I shall give the history of a 

 few of the most interesting species, which have been the 

 objects of special observation. 



It will be necessary however to premise that, in common 

 with the Actinoid Order, the tentacles of the Hydroida are 

 furnished with a kind of stinging weapon, in the form of 

 very minute poisoned darts, which can be projected from 

 capsules embedded in the tissues. By means of these the 

 Hydroida can not only make their tentacles adhere to the 

 victims, but also benumb and paralyze the latter so as to 

 diminish their chances of escape. 



Hydractinea echinata. 

 A little creature, about one-third of an inch in length, with 

 a club-shaped head and a ring of tentacula, living on old 

 shells in deep water. Each Hydractinea is independent ; 

 but they live in numbers on the same shell. They are said 

 to be partial to the same shells, such as Buccimm undatum, 



