TRACHYMEDUSAE 45 



small than large tentacles, whereas in all specimens above 17 mm. in diameter large tentacles are more 

 numerous than small ones. 



The statocysts are as described by Russell. They are internal vesicles inside the perradial and inter- 

 radial marginal swellings immediately at the base of the dwarf tentacle. According to Russell the endo- 

 dermal root of the dwarf tentacle divides into two ; I have found it divided into three branches, one 

 on each side and one in front of the vesicle (PI. V, figs. 2-3). When seen from the exumbrellar side, 

 these branches may appear as 1-3 small protuberances above the outline of the swelling; this is what 

 Bigelow has seen and figured in PI. 30, fig. 5. 



Evidently the eight small tentacles situated on the perradial and interradial swellings never develop 

 into large tentacles; even in the largest specimens they are very small, usually without nematocyst 

 knobs, though sometimes with a small terminal knob, rarely also with a small cluster of nematocysts 

 inside the terminal knob. Thus these eight small tentacles may really be designated as dwarf 

 tentacles in an arrested stage, whereas all the other small tentacles may develop into large ones. 

 Following Bigelow and Russell, therefore, one may regard each of the large perradial and interradial 

 tentacles with its attendant dwarf tentacle as a 'group', which justifies the reference of this species 

 to the genus Gossea. 



Distribution. Acapulco Harbour, Pacific coast of Mexico (Bigelow); coast of Louisiana, Gulf of 

 Mexico, in brackish water (Russell); eastern entrance to the Strait of Magellan ('William Scoresby '). 



TRACHYMEDUSAE 



Family Ptychogastridae 



Ptychogastria polaris Allman 1878 



1878 Ptychogastria polaris Allman, p. 290, 3 figs. 



1881 Pectyllis arctica Haeckel, p. 11. PI. 3, 4. 



1903 Ptychogastria polaris Browne, p. 24. PI. 4, figs, i, 2; PI. 5, figs. 6-8. 



1912a Ptychogastria opposita Vanhoff'en, p. 386. PI. 25, fig. 6. Text-fig. 20. 



1947 Ptychogastria polaris Kramp, p. 4. PI. i, figs. 1-4; PL 6, figs. 1-2. 



Occurrence: St. 177. 5. iii. 27. 63° 17' 30" S, 61° 17' W. Twenty-seven miles south-west of Deception Island, 



South Shetlands. Net: DLH 1080 m. 6 specimens. 

 St. 1958. S-ii. 37. 61'' 17' 54" S, 52° 50' 48" W. East of the South Shetland Islands. Net: DRR 830m. i specimen. 



Vanhoffen gave a beautiful coloured drawing of the antarctic form of Ptychogastria {P. opposita), of 

 which six specimens were taken near the Gauss Station. He gave no real description, but only some 

 remarks for comparison between the arctic and the antarctic forms, which he found very similar to 

 each other, and he erected the new species opposita mainly on account of its great geographical separa- 

 tion. The Discovery specimens from St. 177 are 8-10 mm. in diameter and much contracted. All the 

 tentacles are lost, but their grouping around the umbrella margin can be distinctly seen. The specimen 

 from St. 1958 is fairly well preserved, though here also most of the tentacles are broken off. It is 

 12 mm. wide, only slightly contracted, so that the 16 meridional ridges on the exumbrella are faintly 

 indicated. There are about 48 groups of tentacles; the male gonads are well-developed. The umbrella 

 is more transparent than usually seen in this species, and the anatomical details of the medusa can 

 therefore be distinctly observed. According to a note on the label the central part of this specimen 

 was a rich crimson. 



I have compared the specimens with specimens from the Arctic, and I cannot see any distinguishing 

 features. I think therefore, that we may safely unite the two forms and designate Ptychogastria polaris 

 as a bipolar species. The medusae were taken in dredges hauled along the bottom of the sea, and in 

 the specimens from St. 177 numerous grains of dark sand are found in their subumbrellar cavities; 



