148 AN INTRODUCTION TO ZOOLOGY. 



The Cnidaria are characterized by peculiar stinging 

 cells, or nematocysts (thread-bags), developed from 

 certain cells called Cnidoblasts (nettle-formers). 

 Each of these contains a long thread coiled up, which 

 is sent out to its fnll length to inflict the sting. The 

 use of these is to paralyse the prey on which these 

 animals subsist. The stinging powers of certain jelly- 

 fish, well known to bathers, have procured them the 

 name of Acalepha, or Sea-nettles. Like that of the 

 vegetable nettle, the poison of the sea-nettles aflfects 

 some skins very much more than others. 



Many of the forms belonging to this group are 

 fixed; hence their radial symmetry (see p. 82), which 

 is expressed, so far as their outward form is concerned, 

 by the arrangement of their tentacles, which sur- 

 round the mouth. These tentacles are arms, capable 

 of being extended at length, or retracted into a small 

 space, and provided with numbers of the stinging cells 

 already described. When they touch anything, they 

 retract, carrying the object with them as a rule, and 

 conveying it to the mouth, if it is suitable for food. 

 In the Actinozoa and in Hydra these tentacles are 

 round, and contain a cavity that communicates with 

 the internal gastric cavity. 



Many Ccelenterata have a two-layered ciliated larval 

 stage called a Planula,^ which after a time usually fixes 

 itself to some object, and thus gives rise to a fixed form. 



' So called because by means of its cilia it is able to move 

 about (G-k. Tvkavaofxai, to wander about). 



