44 LIFE BY THE SEASHORE. 



called hydroid, from their general resemblance to the little 

 fresh-water Hydra. 



Among the zoophytes and sea-firs the character which 

 varies most, and which affords a basis for classification, is 

 the skeleton. Let us first understand what this skeleton 

 is, and what is its function. Both the terms "skeleton" 

 and " supporting substance," which one naturally applies to 

 it, are misleading, because both suggest the idea of support. 

 Now the sea-nettles do not require support for their soft 

 parts, because these can be, as it were, stretched by the 

 water which the animals take into their central cavity. 

 An anemone when extended, i.e. when filled with sea-water, 

 is firm and tense ; it is only when it ejects this water that 

 it collapses. The main object of the skeleton in those sea- 

 nettles which possess this structure must therefore be to 

 serve as a means of protection. Take our little pink 

 Hydradinia, for example. When alarmed the zooids con- 

 tract and cower down among the spines, so that an in- 

 quisitive foe darting at the floating pink things will find 

 them lost among these hard inedible thorns. Again, look 

 at any of the common sea-firs so frequent on weed. The 

 tiny branches are crowded with zooids, possibly edible 

 enough, but each of these is placed in a little cup of horny 

 matter. When alarmed they withdraw into the cups, 

 and a persevering enemy is likely to get a maximum of 

 indigestible horn, and a minimum of digestible zooid. 

 Zoophytes and sea-firs are extraordinarily numerous on the 

 shore rocks, and in most cases they are protected to a 

 greater or less extent by a horny skeleton. " , 



In the minority the skeleton is represented either by a 

 mere plate at the base of the colony, or by tubes which 

 envelop some part of the columnar body. In these naked 

 forms (Gymnoblastea) the individuals are often large and 

 highly coloured with numerous conspicuous tentacles. In 

 the majority of the shore forms the skeleton is greatly de- 

 veloped, and carries little cups, in which the zooids are 

 placed, and into which they can be completely retracted. In 

 this set, called Calyptoblastea from the cups, the individuals 

 are small, but usually very numerous, and the skeleton is the 

 most conspicuous part of the colony. 



We shall mention the salient characteristics of a few of 



