CCELENTERATA : HYDROZOA. I i 5 



fixed. When detached, the animal is able to swim with toler- 

 able rapidity by means of the alternate contraction and expan- 

 sion of the umbrella. Around the margin of the umbrella are 

 tufts of short tentacular processes, and in its centre is a poly- 

 pite with a quadrangular, four-lobed 

 mouth. " In transverse section the 

 polypite may be described as some- 

 what quadrilateral, with a sinuous out- 

 line which expands at its four angles 

 to form as many deep longitudinal 

 folds, within which the simple gener- 

 ative bands are lodged " (Greene). 

 Wide longitudinal canals are formed 

 by septa passing from the walls of the 

 polypite to the inner surface of the cup, 

 and a circular canal runs immediately 

 beneath the insertion of the tentacles. 

 The reproductive elements are pro- 

 duced within the body of Lucernaria 

 itself, without the intervention of any 

 generative zooid. 



ORDER II. PELAGID^E. This order 

 is defined as including Lucernarida 

 which possess a single polypite only, and 

 an umbrella with marginal tentacles. 

 The reproductive elements " are developed 

 in a free umbrella, which either consti- Fig. 32 . Lucemariadae. z- 

 tutes the primitive hydrosoma, or is STT^ffStSSS 

 produced by fission from an attached Johnston). 

 Lucernaroid" (Greene). 



Two types, therefore, exist in the Pelagida. The one type 

 is represented by a fixed " trophosome," resembling Lucernaria, 

 but distinguished from it by the fact that the generative ele- 

 ments are not developed in the primitive hydrosoma, but in a 

 free " gonosome," which is produced for the purpose. The 

 second type, represented by Pelagia itself, is permanently free, 

 thereby differing from Lucernaria, which it approaches, on the 

 other hand, in the fact that its generative elements are pro- 

 duced in its' own umbrella without the intervention of free 

 generative zooid s. Pelagia, however, differs considerably in 

 structure from Lucernaria, and in all essential characters is not 

 anatomically separable from a Stegatwphthalmate Medusid. 

 The process of reproduction as displayed in the first section of 

 the Pelagidcz will be considered when treating of that of the 

 Rhizostomida, there being no important difference between 



