HYDROMEDUSAE 



307 



secondary lobes growing out along the proximal part of each of the two canals, until in 

 the adult stage the stomach has eight radial lobes of equal size and appearance and 

 separated by equal incisions (Fig. 5). The development of the stomachal lobes is 



B 



2IM ISMElM lEM 2M I 



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Fig. 6. Willia nmtabilis. A series of figures showing the development of the branches 



of the radial canals, and the variations in the order of arrangement. 



illustrated in Fig. ']a-b. The central cavity of the stomach is not subject to a corre- 

 sponding secondary division, but is always cross-shaped in transverse section; even 

 in the adult, where the eight peripheral lobes 

 are fully and equally developed, the central cavity 

 has only four longitudinal ridges. The rate of 

 development of the stomachal lobes does not 

 strictly follow that of the radial canals; eight 

 distinct lobes may be present in specimens with 



only two to three branches to each main radial pjg .^ p^,y/,-^ mutabilis. The stomachs of two 

 canal, but in others with three to four branches intermediate stages: a, with four undivided 

 to the main canals the division of the four lobes ; 6, with four bifurcated lobes, 

 primary lobes of the stomach is far from being completed. 



(2) Specimens with six main radial canals, regularly arranged (twenty-two specimens 

 examined). The mode of branching of the radial canals is entirely the same in the 

 six-rayed specimens as in those with eight main radial canals, but the final number 

 of branches to each main canal is larger, six or more. Apparently the six main radial 

 canals as well as the six stomachal lobes are all of equal origin (as in Willia stellatd), but 

 this cannot be decided with certainty, as no very young specimens of this group are 

 present. The youngest (16 January 1902) has already two branches to each of the 

 main canals; it is i|mm. in diameter and has twelve tentacles and six small basal 

 bulbs. Ten specimens, i J-3 mm. wide, have three branches to each main canal ; some 

 of them have twelve tentacles and twelve small bulbs, others have twenty-four tentacles. 



6-2 



