230 ROBERT PAYNE BIGELOW ON 



There has been considerable discussion in regard to the nature of stabilization; the 

 question being whether the medusa is to be regarded as a metamorphosed scyphistoma, 

 or as derived from the scyphistoma by a process of budding. In the monodiscous strobila 

 of Cassiopea we have clearly a metamorphosis. The form of the medusa is the result of a 

 series of changes which begin very early and involve all the essential organs of the scy- 

 phistoma. The portion not involved in these changes merely serves as a mechanical sup- 

 port. That this part is separated off and regenerates the lost parts, instead of being 

 absorbed, may be regarded as merely an incidental fact. 



If there be any question of budding it refers to this basal segment. And this suggests 

 a striking analogy between the basal polyp and the peculiar planula-like buds. In the 

 first place they have the same orientation relative to the upper disc. In both, the distal 

 end forms the stem and the proximal end forms the mouth. In the second place, they 

 have essentially the same structure. Each one consists of a simple sac with a wall made 

 up of three layers, ectoderm, mesogloea and endoderm, and each is provided with four longi- 

 tudinal muscles imbedded in the mesogfoea. What differences appear in the subsequent 

 development, may be attributed to the different ways in which the two become separated 

 from the disc and to the greater size of the longitudinal muscles in the basal polyp. The 

 production of supernumerary tentacles, rhopalia, and marginal lobes, is common in this 

 species. Why may we not regard the buds as supernumerary basal polyps, and their 

 subsequent development as a process of regeneration preserved and modified by natural 

 selection for its obvious advantage? 



The ephyrula of Cassiopea has the same number of rhopalia as the adult, and differs 

 in shape from the corresponding stage of ordinary scyphomedusae with eight parameres. 



The most important event in the later stages is the metamorphosis of the mouth 

 parts. The angles of the four quadrate lips become extended to form eight oral arms 

 somewhat similar to those found in the adult Aurosa. There is no Archirhiza stage, but 

 there follows a stage with the oesophagus divided into four tubes, and with three oscula 

 and an oral vesicle on each arm. A similar stage has been found in Pilema and Coty- 

 lorhiza, and it may have some phylogenetic significance. 



The studies of numerous investigators upon the Ascidians have demonstrated that a 

 knowledge of the gamogenetic development of an animal will not always enable one to 

 predict how the organs will be formed in the agamogenetic process. So it may be 

 objected that the results set forth in this paper do not afford a valid basis for the criticism 

 of work done by others on larvae developed from the egg. On the other hand, the eight- 

 tentacle stage of the bud larva of Cassiopea is so like the same stage of the sexually pro- 

 duced larva of its near relative Polyclonia that it would be impossible to tell them apart, 

 and, in the absence of any evidence to the contrary, there seems to be no necessary reason 



