55 MANUAL OF ZOOLOGY. 



that the thread-cells have the power t>f piercing and irritating 

 the human skin but even in the diminutive Hydra it is 

 probable that they exercise some benumbing and deleterious 

 influence on the living organisms which may be captured as 

 prey. Besides the thread-cells, the tentacles of some Hydroids 

 are furnished with rigid hair-like processes, which are probably 

 tactile in function, and which are known as " paipocils." The 

 Cczlenterata are divided into two classes, termed respectively 

 the Hydrozoa and the Actinozoa. 



CLASS I. HYDROZOA. 



The Hydrozoa are defined as Ccelenterata in which the walls 

 of the digestive sac are not separated from that of the general 

 body-cavity, the two coinciding with one another ; the reproductive 

 organs are in the form of external processes of the body-wall. 

 (Fig. 1 8, a, b.) 



It follows from the above, that, since there is but a single 

 internal cavity, the body of a Hydrozoon on transverse section 

 appears as a single tube, the walls of which are formed by the 

 limits of the combined digestive and somatic cavity. 



The Hydrozoa are all aquatic, and the great majority are 

 marine. The class includes both simple and composite organ- 

 isms, the most familiar examples being the common Fresh-water 

 Polype (Hydra), the Sea-firs (Sertularida), the Jelly-fishes 

 (Medusa), and the Portuguese man-of-war (Physalia). Owing 

 to the great difficulty which is ordinarily experienced by the 

 student in mastering the details of this class of animals, it 

 has been thought advisable to introduce here a short explana- 

 tion of some of the technical terms which are in more general 

 use in describing these organisms. 



GENERAL TERMINOLOGY OF THE HYDROZOA. 



Individual. We have already seen (see Introduction) 

 the term "individual," in its zoological sense, must 

 stricted to " the entire result of the development of a 

 fertilised ovum," and that in this sense an individual ^kijay 

 either be simple, like Amoeba, or may be composite, like a 

 Sponge, which is produced by an aggregation of amoebiform 

 particles. If all the parts composing an individual remain 

 mutually connected, its development is said to be " continu- 

 ous ; " but if any of these parts become separated as inde- 

 pendent beings, the case becomes one of "discontinuous" 

 development. We have seen, also, that however long zooidal 



