142 ZOOLOGY SECT. 



or float freely on the surface of the ocean, and such pelagic species 

 are always found to exhibit a remarkable degree of polymorphism, 

 the zooids being of very various forms and performing diverse 

 functions. 



Thus we have zoophyte colonies known to produce free medusae, 

 zoophyte colonies known not to produce free medusae, and medusae 

 known to have no zoophyte stage. Moreover, there are many 

 medusae of which the life-history is unknown, so that it is 

 uncertain whether or not a zoophyte stage is present. It is also 

 found that in some cases closely allied zoophytes produce very 

 diverse medusae, while similar medusae, in other cases, may spring 

 from very different zoophytes. For these reasons a sort of double 

 classification of the Hydrozoa has come about, some zoologists 

 approaching the group from the point of view of the zoophyte, 

 others from that of the medusa. On the whole the following 

 scheme seems best adapted for bringing before the beginner the 

 leading modifications of the class. 



ORDER 1. LEPTOLINSE. 



Hydrozoa in which there is a fixed zoophyte stage, and in which 

 the sense-organs are exclusively ectodermal. 



Sub-Order a. Aniliomedusce. 



Leptolinse in which the polypes are not protected by hydrothecs; or the 

 reproductive zooids by gonothecae : the medusae bear the gonads on the 

 manubrium and have no lithocysts. 



Sub-Order b. Leptomedusce. 



Leptolinse in which hydro- and gono-thecae are present : the medustc bear 

 the gonads in connection with the radial canals and usually have lithocysts. 



Hydrozc 



ORDER 2. TRAOHYLIN^E. 



ioa in which no fixed zoophyte stage is known to occur, 

 all members of the group being locomotive medusae, some of which 

 have been proved to develop directly from the egg. The sense- 

 organs are formed partly of endoderm. 



Sub-Order a. Tracliymedusce. 



Trachylinse in which the tentacles spring from the margin of the umbrella, 

 and the gonads are developed in connection with the radial canals. 



Sub-Order b. Narcomedusce. 



Trachylinae in which the tentacles spring from the ex-umbrella, some 

 distance from the margin, and the gonads are developed in connection with 

 the manubrium. 



