224 ROBERT PAYNE BIGELOW ON 



I 



movements are rapid and violent. The rhythm is interrupted by few pauses, and these 

 are short. The result of these movements is that the thin wall of the isthmus is 

 ruptured, and the ephyrula is set free. 



After this separation, the basal polyp has the appearance represented in Figs. 27 and 

 28. It is a scyphistoma with seventeen short tentacles and a rudimentary proboscis 

 (Figs. 61 and 62). The proboscis and the tentacles grow rapidly, so that in a few days 

 it is impossible to distinguish a regenerated basal polyp from a young scyphistoma in the 

 sixteen-tentacle stage, except that the former has a somewhat thicker stem. It may be 

 inferred from this complete regeneration of the basal polyp that it undergoes repeated 

 strobilization, as Glaus 1 has found to be the case in Aurelia. 



The Ephyrula. The ephyrula of Cassiopea is very different in appearance from the 

 corresponding stage in ordinary scyphomedusae with eight rhopalia. Cotylorhiza has an 

 ephyrula resembling the same stage in the semostomatous medusae. Good figures of 

 this are given by du Plessis and Glaus, and there is a striking difference between these 

 figures and Figs. 29 and 30 in this paper, which are camera drawings of well-preserved 

 ephyrulas of Cassiopea, mounted in balsam. Fig. 29 represents a young Cassiopea that 

 has not long enjoyed a free existence. The general shape of the umbrella is like that of 

 the adult, and there is the same concavity in the centre of the exumbrella, while the 

 margin curves in the opposite direction, as in Fig. 64. The typical ephyrula of Aurelia 

 or Cotylorhiza has eight marginal arms with two lobes at the end of each, and between 

 each pair of lobes there is a rhopalium. In Cassiopea structures corresponding to these 

 arms are present to the number of sixteen, or often more. But these do not destroy the 

 general circular outline of the animal, for they are connected by thin areas on the 

 umbrella, alternating with an equal number of ridges, which at an earlier stage bore the 

 interrhopalial tentacles on their under sides. 



We have, then, at this stage the marginal zone of the umbrella marked by a number 

 of short radial ridges separated by an equal number of thin areas. The ridges are in 

 line with the radial canals. At the peripheral end of each ridge the margin of the 

 umbrella is produced into two lobes, those adjoining the rhopalia being well marked, 

 the others small and inconspicuous (il. Fig. 30). 



In Fig. 29 there are seventeen, and in Fig. 30, twenty-three, rhopalia. The latter 

 is an unusually large number, and it will be noticed that the number of marginal lobes 

 has not increased in proportion, so that irregularities of the margin occur in many places, 

 as described in the section on variations. 



At this stage the rhophalia have come to lie, as in the adult, wholly within the margin 

 of the umbrella, and project from its subumbrellar surface. The interrhopalial tentacles 



1 See foot-no te,_Claus ('92). 



