SCYPHOMEDUSAE 



371 



tional stages, however, between the narrow flat furrows of the verrilli type and the broad 

 deep ones of the ivyvillei type. 



Broch, who studied the rich and fresh material of the Michael Sars Expedition (± 200 

 specimens) separates the specimens of bairdi into three groups according to the radial 

 furrows: one group provided with distinct radial furrows all over the central disc, a 

 second with incomplete furrows visible only at the margin of the disc, and a third with 

 a perfectly smooth disc without any traces of radial furrow. (The only specimen caught 

 by the ' Michael Sars ' with deep broad notches and furrows on the pedalia was identified 

 by Broch as A . zvyvillei.) Broch 's first group of bairdi comprises no less than one-third of 

 his material. In the present series, on the other hand, there is not a single specimen with 

 radial furrows running all over the disc. The central portion is always smooth; the 

 furrows are restricted to the marginal region of the central disc, with the exception only 

 of a few specimens showing a structure similar to a network with large regular or 

 irregular polygonal meshes (Sts. 86, 405 ; Fig. 3). One may think that during the haul 

 the net has left an impression of the 

 meshes on the soft surface of the central 

 disc. In those cases the indentation of 

 the margin was never radial, as is usual, 

 but irregular, with a variable number 

 of notches. In other cases there were 

 on the surface of the disc four furrows 

 which formed a cross, with a depression 

 at the apex, by which the surface was 

 divided into four parts. Broch, with 

 his fresh material, succeeded in pro- 

 ducing the furrows running over the 

 disc by a slight pressure with the point 

 of the forceps in the middle of the disc, 

 and believes that the radial furrows of 



the bairdi type may be due to contractions of the central disc. I repeatedly tried to pro- 

 duce radial furrows by a similar procedure, but in vain, perhaps because my material is 

 mostly several years old. With regard to the other groups Broch mentions less than one- 

 third of all Atolla as being of the verrilli type (fifty-seven) and a little more than one- 

 half (103) of the bairdi type. I found nearly as many specimens of the bairdi as of the 

 wyvillei type (seventy-two and ninety-one respectively) and relatively few (twenty-one 

 specimens) of the verrilli type. 



Certainly the radial furrows on the central disc do not attain their typical broad shape 

 before the specimen has reached a considerable size, being broad in large, and narrow 

 in small specimens. In the large Antarctic specimens from Sts. 41 , 42, 44, 45 the furrows 

 are very conspicuous: large, long, deep and, moreover, darkly pigmented. In some 

 specimens a part of the furrow is broad and another part much narrower, and the same 

 applies to the pedalia. 



Fig. 3. Atolla wyvillei, Haeckel, forma bairdi, mihi. 



Central disc with rough {a, b) or finer (c) network. 



Three specimens from St. 86, x |-. 



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