1046 



MR. C. L. BOULENGER ON 



and tentacles in 400 medusje taken at random from the mateiial 

 at my disposal : — - 



Table shoioing the Number and Arrangement of the Radial Canals 

 and Tentacles in 400 individaals. 



It will be noticed that the 55 abnormal individuals fall naturally 

 into two well-marked groups : — («) which includes those medusaj 

 which deviate from the normal tetramerous symmetry ; and (6), 

 which includes medusje with the normal number of radial canals 

 and primary, perradial tentacles, but possessing, in addition, 

 secondary tentacles situated between the four primary ones and 

 not connected with the stomach by means of radial canals. 



It is my intention to discviss these two groups separately, as 

 it is obvious that in them we are dealing with two completely 

 distinct phenomena. 



(a) The normal medusa of Moarisia lyonsi is provided with 

 four I'adial canals at the distal extremities of which the four 

 perradial tentacles are given oft'. The gonad-bearing region of 

 the stomach is produced into four perradially situated pouches 

 which in the adults extend as finger-shaped diverticula for a 

 considerable distance along the radial canals. 



Eleven specimens out of the 400 medusse examined {i. e. 2-75 

 per cent.) were found to deviate from this general tetramerous 

 symmetry ; one medusa possessed only three radial canals, three 

 tentacles and three gonadial diverticula ; nine medusfe had five, and 

 one had six of these structures. This type of variation is known 

 to occur in many species of medusse which are normally tetra- 

 merous in symmetry ; among craspedote forms it has been studied 

 best in Obelia {Eucope) [2], Clyiia [3J, Sarsia [4, 6], Rathkea 

 {Lizzia) [7j, Podocoryne [8], Gonionemus\S\?inA Limoiocnida [9]. 



