SPONGES, ZOOPHYTES, AN f D SEA-FIRS. 43 



From this description of Hydradinia a general idea of 

 the character of the Coelentera may be gathered. The 

 members of the group are usually either polypes, like those 

 of Hydradinia, or are jelly-fish, like the swimming-bells of 

 many zoophyte colonies ; but both types of structure occur 

 in many much-modified forms. Both types not infrequently 

 occur in the course of one life-history, and then the phe- 

 nomenon which we have already studied as alternation of 

 generations is produced. Many forms are colonial, like 

 Hydradinia, and in such colonies there may be division of 

 labour among the members of the colony. 



The Coelentera are very numerous, and are found in all 

 seas and at all depths ; but the different parts of the ocean 

 have their characteristic forms. Thus, as we all know, the 

 reef-building corals are confined to the warm seas, and even 

 in the British area there are far more sea-anemones on the 

 South and West than in the colder waters of the East 

 Coast, while certain zoophytes which occur in the North and 

 East are absent in the South and West. 



In studying the Coelentera we shall begin with the zoo- 

 phyte colonies, similar to Hydradinia, which are so abund- 

 ant on our coast. Of these, Hydradinia is, in one sense, 

 a relatively simple type, for its skeleton is only represented 

 by the crust which covers the shell on which the colony is 

 placed, and by the little spines arising from this crust. In 

 most of the zoophytes the colony is surrounded by a pro- 

 tective sheath, which sometimes forms little cups in which 

 the individual zooids are placed. As the sheath is tough 

 and resistant, it not only keeps the colony expanded during 

 life, but also persists after the death of the zooids. These 

 dead colonies are often flung up on the beach, and are more 

 familiar to most people than the living zoophytes. From 

 their peculiar method of branching they are known as " sea- 

 firs," or are often incorrectly regarded as " seaweed." The 

 first class of Coelentera, therefore, includes delicate zoo- 

 phytes, with practically no skeleton, such as Hydradinia 

 and many others ; the sea-firs, with their resistant coat ; 

 the swimming-bells, or medusoids, which arise from many 

 zoophytes and sea-firs; and also some other colonial forms 

 much less common in our seas. This first class is termed 

 the HYDROZOA, and the individual zooids, or polypes, are 



