182 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The sexual organs of the larger jellyfishes (Scyphomedusae) are 

 entodermal and are found in the stomach, peripheral to, and closely 

 associated with, the four clusters of gastric cirri. The sexes are 

 usually separate, the animals being either male or female, although, 

 in rare instances, as in Chrysaora, they are hermaphroditic, or male 

 when young and female when old. When mature at the breeding 

 season the males and females usually come to the surface in great 

 numbers and may congregate in vast swarms many square miles in 

 area. The larvae or eggs may then be cast out into the water by the 

 breaking down of the stomach wall, or the larvae may undergo a 

 part of their development within the stomach, or mouth parts, of 

 the mother, finally to be cast out through the mouth, or set free 

 from the disintegrating bodies of the dying parents. 



In any event the larvae soon develop into minute pear-shaped 

 creatures about as large as a pin's head, their bodies being covered 

 with vibrating cilia, which enable them to spin and progress through 

 the water. For a few days, or even weeks, they may remain thus 

 swimming near the surface and may be drifted hundreds of miles by 

 tide and ocean current. Soon, however, the little pear-shaped planula, 

 as it is called, settles down head first upon the bottom and fastens 

 itself to some fixed object. Then for the first time the mouth devel- 

 ops at that which was the posterior end of the planula, and tentacles 

 grow out so that the mouth is soon surrounded by 16 or more of these 

 organs which serve to capture the minute Crustacea and other organ- 

 isms upon which the little polyp feeds. Thus it remains sedentary 

 for a long period, growing all the time and superficially resembling a 

 small sea-anemone. Finally a series of constrictions develop at reg- 

 ular intervals around the sides, and the creature appears as if it were 

 composed of a series of disks set one upon the other. The margin of 

 each disk soon develops eight cleft lobes, and eight sense-clubs appear 

 in the clefts. Then the uppermost disk, containing the mouth and 

 the crown of tentacles, is cast off and perishes, while the others are 

 set free in succession and swim away as minute jellyfishes, soon to 

 develop tentacles and finally to become mature and repeat this pe- 

 culiar process of development. After the last disk has been cast off, 

 only the stump of the strobila, as it is called, remains, but this may 

 regenerate a new ring of tentacles and continue to grow, and possibly 

 to develop more jellyfishes at the succeeding season. 



There are many interesting variations of this typical process of 

 development. Often the strobila, instead of giving off a series of 

 disks, develops only a single constriction, and every alternate tentacle 

 changes into a sense-club, while the other tentacles may be wholly 

 absorbed, so that they disappear. In this case only a single ephyra 

 or larval jellyfish is set free. This form of development is especially 

 characteristic of the Rhizostomae or multi-mouthed jellyfishes, such 



