108 SYNOPTIC COLLECTION. 



Aurelia generally develops with alternation of genera- 

 tions, as above described, but in isolated cases the devel- 

 opment is accelerated, the hydroid stage being omitted, 

 and the medusa develops at once from the egg. 



In Pelagia noctiluca Per. & Less. (No. 137), the sessile 

 hydra stage is always omitted and the parenchymella de- 

 velops without intermediate forms into the medusa. This 

 process is not comparable with the simple, primitive de- 

 velopment of ancestral forms, nor with the direct develop- 

 ment of Aeginopsis; neither can it be compared with the 

 indirect development peculiar to those forms which pass 

 through an alternation of generations, nor with the sup- 

 pressed development observed in Hydra. It is rather an 

 illustration of accelerated development which character- 

 izes not the primitive but the secondary and specialized 

 members of a group. 



A more complicated condition is found in Rhizostoma 

 pulmo Linn. (Nos. 138, 139) in which the margins of the 

 lips have become united so that the food is taken in 

 through a large number of minute openings in the tenta- 

 cles. 



Haliclystus auricula Clark (No. 140), is our common 

 Lucernarian. It is a beautiful green medusa about an 

 inch in diameter and is fastened temporarily by a 

 sucker on the smaller end of its body. The habit of 

 creeping peculiar to the adult Haliclystus has become 

 fixed in the young, so that the latter is not free-swimming 

 but crawls over eel grass from an early age. This is also 

 true of the young of Lucernaria, which is not provided 

 with cilia but creeps over surfaces. The adult Lucernaria 

 (No. 141) is divided into eight lobes. The cavity of the 

 oral cone communicates with a central chamber whence 

 four wide chambers pass into the lobes. 



