CCELENTERATA. 163 



provisional hydrophyllium. This arises as a prominence at the aboral pole, 

 containing a prolongation of the gastrovascular cavity. Between the epiblast 

 and hypoblast of the prominence gelatinous tissue becomes deposited, and 

 the hydrophyllium is thus converted into a large umbrella-like organ enclosing 

 the polypite. The two together have a close resemblance to an ordinary 

 Medusa, the polypite forming the manubrium, and the hydrophyllium the 

 umbrella. The hydrophyllium is eventually thrown off. 



An important type of Physophorid development is exemplified in Crystal- 

 loides, a genus closely allied to Agalma. In this type the greater part of the 

 original ovum, instead of directly giving rise to the polypite, becomes a kind 

 of yolk-sack, from which the polypite is secondarily budded (fig. 77, yk). 

 Agalma sarsii is in this respect intermediate between Crystalloides and 

 Physophora. Both these types are remarkable for developing a series of 

 provisional hydrophyllia (fig. 77, h.ph^. In both genera the first of these 

 develops as in Physophora, and for a long time is the only one functional. 



The conclusions to be drawn from the above description may 

 be summed up as follows. In all the Siphonophora, so far 

 observed, the starting-point for further development is a typical 

 ciliated two-layered planula. The inner layer or hypoblast is 

 mainly formed of large nutritive cells. From these cells an 

 epithelial hypoblastic layer becomes secondarily differentiated, 

 the exact relations of which differ somewhat in the various 

 types. The nutritive cells themselves do not appear to become 

 directly converted into the permanent hypoblastic tissues. The 

 development of the adult from the planula commences by the 

 thickening of the epiblastic layer, usually at one pole (the future 

 proximal or aboral pole), and the formation at this pole of a 

 series of bud-like structures (in the growth of which both 

 embryonic layers have a share), which become converted into 

 the hydrophyllia, nectocalyces etc. The main oral part of the 

 planula becomes generally converted into the polypite, though 

 in some instances (Crystalloides) it remains as a yolk-sack, and 

 only secondarily gives rise to a polypite. 



Two very different views have been taken as to the nature of 

 the various component parts of the Siphonophora, and the 

 embryological evidence has been appealed to by both sides in 

 confirmation of their views. By Huxley and Metschnikoff the 

 various parts nectocalyces, hydrophyllia, hydrocysts, polypites, 

 generative gonophores etc. are regarded as simple organs, while 

 by Leuckart, Haeckel, Claus etc. they are regarded as so many 

 different individuals forming a compound stock. The difference 



II 2 



