HYDROMEDUSAE 301 



It is difficult to draw definite boundaries to mark off the different stages, except in 

 the early stages where the number of tentacles may be taken as a guide and three 

 divisions formed: 



First stage, diameter of umbrella o-6 0-9 mm. Four perradial tentacles and four 

 interradial basal bulbs ; eight adradial marginal vesicles. 



Second stage, diameter about 07-1 -2 mm. Eight tentacles (four perradial and four 

 interradial), eight small basal bulbs developing close beside the eight adradial marginal 

 vesicles. 



Third stage, diameter about 2-0-2-5 mm. Sixteen tentacles (four perradial, four 

 interradial, and eight adradial), sixteen small eradial bulbs just appearing, sixteen 

 marginal vesicles. 



After the third stage come the intermediate stages, and it is practically impossible to 

 classify them into divisions. The number of tentacles is very variable and is not corre- 

 lated with the size of the umbrella, nor the size of the gonads. When the umbrella is 

 about 5 mm. in width, the gonads are about 2 mm. long, situated near the margin of the 

 umbrella; about twenty to thirty tentacles and nine to twelve tentacular bulbs; one 

 marginal vesicle between every two tentacles or bulbs. The intermediate stages may 

 be characterized by having immature gonads and more than sixteen tentacles. It is 

 only by the state of the gonads that it is possible to tell when the adult stage is reached ; 

 and the gonads, even when ripe, do not indicate that the medusa has reached its 

 maximum growth. 



Abnormal specimens. In the first collection one specimen was observed with three 

 radial canals and three gonads ; stomach triangular, mouth with three lips. Thirty-two 

 tentacles, and twelve tentacular bulbs. Umbrella 13 mm. in diameter. In the second 

 collection there is one specimen, 4 mm. wide, with six radial canals and six gonads; 

 the space between two of the radial canals is somewhat narrower than the other five 

 spaces. There are twenty-one tentacles, eighteen tentacular bulbs, and twenty-two 

 marginal vesicles. 



Young and intermediate stages as well as adult specimens were found together at any 

 time during the period from the beginning of November igoi to the end of March 1902. 



Ph. simplex, which is a very common medusa at the Falkland Islands, has recently 

 been recorded from the Patagonia Bank, east of Rio Grande do Sul on the east coast 

 of South America, about 30° S, where one small specimen, 2 mm. in width, was found 

 by the 'Meteor' Expedition (Thiel, 1938). 



Among the numerous species of Phialidium described, only one species may be said 

 to bear a close resemblance to the Falkland species, viz. Ph. islandiciim Kramp (1919, 

 p. 95). This species occurs in the northern Atlantic, round Iceland and between the 

 Shetland Islands and Norway ; it may grow to a considerable size, more than 40 mm. in 

 diameter, with about 200 tentacles ; moreover, it is distinguished from Ph. simplex by 

 the greater length of the gonads. Intermediate stages of Ph. islandiciim, however, are 

 much like Ph. simplex, and both species are characterized by the number of marginal 

 vesicles being equal to the number of tentacles. 



