1048 MR. C. L. BOULENGER ON 



20'9 °/q and 22 °/o5 respectively, were abnormal, the number of 

 these sense-organs ranging between 6 and 15, the normal number 

 being, of course, 8. These figui-es, however, include a number of 

 specimens which are undoubtedly teratological monstrosities. 



Browne pointed out that variation in the number of tentaculo- 

 cysts does not necessarily interfere with the other organs of the 

 body, but that there is a correlated variation between the number of 

 genital pouches and buccal arms as shewn by eight specimens : — 

 4 individuals had 3 genital pouches and 3 buccal arms, 1 had 5 

 pouches and 5 arms, and 3 had 6 of these organs. 



In Moerisia there must necessarily be a correlation between 

 the number of primary tentacles and radial canals, since recent 

 researches on the development of the gonophores of Hydromedusse 

 [13, 14] have shown that these structures ai-ise together from the 

 endodermal pouches of the young medusa-bud. 



There is also a correlated variation between the number of 

 I'adial canals and gonadial pouches, and this is only what we 

 should expect as the latter structures spread outwards fi'om the 

 stomach on to the radial canals. The single 6-rayed specimen, 

 however, had one gonadial pouch very poorly developed although 

 the other five were quite normal. 



It is interesting to note that vai'iation in the number of 

 primary radii in the medusa does not necessarily aflfect the 

 symmetry of the manubrium ; this point is well shown in the 

 sections figured on PI. LIX. 



A normal medusa with four tentacles, radial canals and 

 gonadial diverticula has the distal part of the manubrium 

 provided with a similar number of conspicuous endodermal 

 ridges or tfeniolse, which can be seen in the section figured 

 (PL LIX. fig. 1); a similar section through a pentamerous 

 individual (PL LIX. fig. 2) demonstrates the fact that, although 

 five canals and gonads are well developed and the manubrium 

 appears almost pentagonal, the number of endodermal ridges is 

 not affected, although their symmetrical arrangement is disturbed. 



The above is in accordance with the observations of other 

 investigators : — Agassiz and Woodworth [2] examined 4000 

 specimens of Ohelia (J^Jucope), but noted no variations in the 

 shape of the digestive cavity or in the number of the actinal lobes 

 of the manubrium, even in specimens with five or six radial 

 canals in place of the normal number (four) the actinal lobes 

 being always found to be four in number. 



A. G. Mayer [3] has also made some interesting observations 

 which bear on this point. This author has made a careful study 

 of the medusa Pseudoclytia pentata, the only Leptomedusan 

 which is normally pentamerous, and which he considers to be 

 derived from some species of Clytia {Epenthis), e.g. C.folleata. 

 In Pseudoclytia pentata there is much variation both in the number 

 of radial canals and in the number of oral lips ; but whilst the 

 former incline towards the production of more than five canals, 

 the oral lips show a decided tendency to revert to the ancestral 

 condition of four. 



