ON A FHEK-SWIMMING HYDEOID. 19 



While these changes have been going- on the original 

 gastral cavity of the bud becomes further subdivided by 

 the union of its inner and outer walls interradially (fig. 21) 

 to form the solid endodermal lamella, thus defining the four 

 radial canals and the circular canal. The ectoderm becomes 

 greatly thickened outside the circular canal, and the tentacles 

 begin to grow out. 



Hitherto ectoderm and endoderm have everywhere re- 

 mained in close contact (figs. 20, 21), but the transparent 

 gelatinous mesogloea now appears and forces the layers apart 

 (fig. 25). About the same time the subumbrellar cavity is 

 developed and the velum is ruptured in the middle (fig. 

 20, w.), giving rise to the mouth of the bell (fig. 23). 



5. Discussion of Results, Relationships, etc. 



Pelagohydra mirabilis is a remarkably interesting 

 organism from several points of view. In the first place it 

 forms an excellent example of adaptation to changed con- 

 ditions of life, showing us how a representative of a group 

 whose members are normally attached, in the hydroid phase, 

 to the ends of fixed stalks may become adapted to a free- 

 swimming pelagic existence. In the second place it exhibits 

 remarkable structural features, especially in the compli- 

 cation of the gastral cavity with its endodermal canals, 

 and the development of the float with its extraordinary 

 supporting membranes. It also has very striking histo- 

 logical peculiarities, showing in this respect a degree of 

 diiferentiation perhaps unequalled in any other hydroid.^ 



As a pelagic member of a typically non-pelagic group of 

 animals we may compare it with Pelagonemertes amongst 

 the Nemertines, Tomopteris amongst the Annelids, and 

 Pelagothuria amongst the Holothurians, and it may 



• Tlie gigantic Brancliioceriauthus impcrator probably resembles 

 Pelagohydra closely in histological features, but requires further investiga- 

 tion (vide Miyajima, ' Journ. Coll. Sci. Imp. University of Tukio,' vol. xiii, 

 p. 235, etc.). 



