VAIIIATION IN A J10LI.YKISII. 1051 



groups :— (i.) including niedusie witli an umbrella-diameter 

 measuring between | and 2 mm., and (ii.) with a diameter 

 measuring from 2jJ- irnn upwards. 



Group (i.) contained 278 individuals of wliicli 39, or about 

 14 °/q, possessed supernumerary tentacles between the four 

 primary ones, whereas of the 122 larger medusie belonging to 

 group (ii.) only 5 were provided with such structures. 



Sexual dimorphism is known to occur in a few sj)ecie3 of 

 medusse,* but I have assured myself that this is not the case 

 with the specimens I am describing; microscopic; exa.mination, 

 by means of sections and whole mounts, showed that all the 

 medusae* of Mverisia in my collection belonged to the male sex, 

 whether bearing secondary tentacles or not. 



It must be obvious from the above account that the kind of 

 variation I have just described is of a nature totally different from 

 that treated of in the first section of this note. We are certainly 

 not dealing with a case of ordinary meristic variation, and the 

 oidy conclusion I can arrive at is that these multitentacidar 

 medusa; form a distinct variety which has arisen as a mutation 

 from the noi'mal form : it will be interesting to discover whether 

 this variety will maintain itself in Lake Qurun. 



Altf)getlier, forty-six f multitentacular specimens have been 

 examined by me ; it was found that the number of secondary 

 tentacles varied considerably. 10 individuals possessed four 

 interradial tentacles as well as the four primary perradiaA ones 

 (text-fig. 223) ; 28 had twelve secondary tentacles, four inter- 

 radial and eight adradial (text-fig. 224) ; and 3 possessed sub- 

 radial tentacles in addition to these, the medusa illustrated in text- 

 fig. 225 (p. 1052) having as many as eight of these structures, two 

 in each quadrant, situated between the inteiradial and adradial 

 tentacles. 



The sketches referred to above, although considerably enlarged, 

 are drawn to scale ; they exhibit the fact that this variety of 

 Mverisia lyonsi follows the general rule for multitenta,cular 

 medusae, in that the number of secondary tentacles increases 

 with the size and age of the individuals. The sequence of 

 the development of the secondary tentacles is also quite normal 

 in the specimens just described. 



Although the majority of the multitentacular specimens of 

 Mosrisia exhibited a perfect radial symmetry and developed 

 their secondary tentacles in a perfectly normal order, a few 

 medusae proved abnormal in this respect and showed marked 

 asymmetry. One specimen had a single interradial tentacle 

 developed between two perradial ones, whilst another bore 

 interradial and adi-adial tentacles also in a single quadrant only. 

 Again, two medusje had these organs developed in two of the 

 quadrants (text-fig. 226), and a third had secondary tentacles in. 



* K. fi'. Stonwtoca dinema Agassiz, and Orchisloma pileus Lesson : sec Mayer (10) 

 pp. Ill and 219. 

 t Two of these specimens were not included in the Table on p 1043. 



