THE CCELENTERATA 47 



Hydrozoa. The next class of ccelenterates we shall sur- 

 vey is that of the Zoophytes, or, more properly, Hydroid 

 zoophytes or " Hydrozoa" The term " Zoophyte " was 

 bestowed upon them long ago, when it was thought that 

 they were something between plant and animal, but it 

 is retained on account of their branching, plantlike modes 

 of growth. 



In anatomy they differ but little from the foregoing, 

 except on one or two points. They form branched colonies, 

 and each individual does not hold a separate entity, but is 

 joined to all the others by what would otherwise be its foot 

 that is, while each polyp is separate in outward form 

 it is a portion of the general fleshy contents (termed the 

 Ccenosarc) of the whole tubular stock, and when one polyp 

 captures prey and digests it the proceeds are for the benefit 

 of the community. 



Then they have division of labour : while some are organ- 

 ised for the capture and digestion of food, others are adapted 

 solely for the duties of reproduction certainly very like the 

 case of a plant, with its separate functions of leaf and 

 flower. 



Further, they represent the interesting feature known 

 as the " alternation of generations." The young set free 

 from a reproductive individual are quite unlike the parent. 

 The young are free-swimming, bell-shaped medusids 

 Hydro-medusids, or small jelly-fish. These in turn have 

 sexual reproductive powers, and may give rise either to 

 more medusids like themselves, or more usually to a form 

 which settles down, attaches to some object, buds, and 

 grows plantlike until a branched stock, like the parent ; 

 and so the life cycle is repeated. 



It is sometimes argued as to which can be termed the 

 " parent " the branched form or the medusid. This we 

 must leave for others to decide. 



This is a brief and, of course, rather incomplete outline 



