230 ROBERT PAYNE BIGELOW ON 



Tlieru liiis been coiisidei-able discussion in regard Id (lie nature of strobilization ; the 

 question bcinu; wliL-tlici- the medusa is to be regardeal as a inctamoriihosed scyphistoma, 

 or as derived I'roni tlie seyphistonia by a i)rocess ot" budiUng. In the nionocUseous strobihi 

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

 series of cliangcs which bct^in very early .iml involve all llic essential organs of the scy- 

 pliistoma. The portion not involveil in llicsc changes merely serves as a mechanical sup- 

 [)oi-t. That this part is separated (jff and ri'generates lljc lost parts, instead of being 

 absoi'bed, may be regarded as niercdy an incidental fact. 



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

 a striking analogy between the basal polyp and the peculiar planidadike liuds. In the 

 first place they have the same orientation relative to the u})per disc. In bodi, the distal 

 end forms the stem and llie pro.ximal end forms the month. In the second [ihice, they 

 have essentially the same structui'e. Each one consists of a simple sac with a wall made 

 up of three layei's, ectoderm, mesogloea and en(loderm,and each is provided with four longi- 

 tudinal muscles indtedded in the mesogloea. What differences ;ip[iear in the sul)se4uent 

 development, may be attributed to the ditferent ways in wiiich the two become separated 

 from the disc and to the greater size of the longitudinal muscles in the basal pol\p. The 

 production of supernumerary tentacles, rho|)alia. and marginal lobes, is coininon in this 

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

 subsequent development as a process of regeneration preserved and modilied b\- natural 

 selection for its obvious advantage? 



The eph3'rula of Cassiopea has the same number of rhopalia as the adult, and differs 

 in shape from the corresponding stage of ordinary scy phomedusae with eigiit parameres. 



The most important event in the later stages is the mefamoi[ihosis of the mouth 

 parts. The angles of the four ipiadrate li|)s 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 disided into foui' tubes, and with three oscida, 

 and an oral vesicle on each arm. A similar stage has lieen found in i'ilema and Coty- 

 lorhiza, and it may have some phylogenetic signilicance. 



The studies of numerous investigators ui)on the Ascidians ha\e demonstrateil that a 

 knowledge of the ganiogenetic development ol an animal will not always enal)le one to 

 predict how the organs will be formed in the ag;unogenetic process. So it may be 

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

 of vvork done by others on larvae developed from ihe egg. (>n tlu' other hand, the eight- 

 tentacle stage of the bud lar\a of Cassio|»ea is so like the sauu' stage of the sexually pro- 

 duced larva of its neai' relative I'olyclonia that it would be impossible to tell them a[)art, 

 and, in the absence oi any I'xick'uce to the coulrary. there seems to be no necessary reascju 



