SCYPHOMEDUSAE 



373 



Septal regions ("false septa ") subdividing the tentacular canals near their bases are 

 seen in all three forms. They are present in most specimens of the wyvillei type too, 

 where their absence would be expected according to Bigelow (1909, p. 40). As may be 

 seen from Figs. 4 and 5 they are of great variability both in size and form, very broad and 

 long in large, small and narrow in small individuals, leaf-, egg-, or spoon-shaped. These 

 false septa become very clearly visible when the subumbrellar pigment has fallen off 

 or been taken away. I therefore do not believe that they are a specific character of great 

 importance. 



Excretory organs. The system of probable excretory organs (discovered by Van- 

 hoeffen in 1903), consisting of eight pores in each principal radius near the perradial 

 corners of the stomach, whose position is marked by eight dark brown oval spots upon 

 the floor of the subumbrella (figured by 

 Maas, 1904, pi. v, fig. 38), has been ob- 

 served by me in several specimens, mostly 

 young ones, from Sts. 405 and 440. 



Pigmentation. Broch makes out that 

 his own results, corroborating those of 

 Bigelow, indicate that the pigmentation 

 increases with deeper water. He possessed 

 the freshly preserved material from the 

 Michael Sars Expedition, all with vivid 

 and fresh colours. Although he found 

 the pigmentation of the individuals subject 

 to great variation, and found every transi- 

 tion stage between hyaline and very dark 

 specimens, he kept four groups of a 

 diff'erent colouring separate, limiting the 

 stages as follows: I, only the stomach 

 and occasionally the gonads containing 

 mented; III, pigment covering other parts of the subumbrella too, the gonads being 

 always visible from the exumbrellar side of the medusa ; IV, the pigmentation so dense 

 that the gonads are quite invisible from the upper side. His table 9 shows the bathy- 

 metrical distribution of 180 specimens oi Atolla bairdi: one specimen from 250 m. is 

 almost hyaline. Of the twenty-nine specimens from 500 m., twenty-three belong to 

 stage I, six to stage II. From 750 m. there are forty-eight specimens, five of which 

 belong to stage I, twenty to stage II, sixteen to stage III and seven to stage IV, which 

 means that three-quarters of all the specimens belong to stages II and III. Down to a 

 depth of 750 m. there is indeed an increase in pigment, but not at depths of 1000- 

 1500 m. At 1000 m. more than half the specimens belong to stage II and one-third 

 to stage III; at 1250m. one-third belongs to stage II, one-fifth to stage III and 

 one-quarter to stage IV; at 1500 m. one-half to stage II, one-third to stage HI and 



Fig. 5 . Atolla wyvillei, Haeckel, forma typica, mihi. 



Part of the gastro-vascular system of an adult 



specimen. From St. 86, x 3. 



pigment; II, the ring muscle also pig- 



