OCR Text |
Show 182 MESSRS. L. MURBACH AND C SHEARER ON [June 16, tentacle-bulbs of Zygoclactyla cyanea, which are not true excretory papilla? in which an opening is present, but probably simple tentacle-buds or spurs. Agassiz (2) has described similar structures in a number of ^Equoi-ids, as Ithegmatodes tenuis, Zygodactyla grcenlandica, uEquorea albida. Possibly in some cases these may be the true excretory papilla? which he has seen but misunderstood, or they may be simple tentacle-spurs, and so entirely different structures ; his descriptions and figures do not make this clear: if they are excretory papilla? he does not recognize their structure and proper position. Milne-Edwards (30) speaks of tubercles on the bell-margin of yEquorea violacea, " ayant la forme d'un petit mamelon " (pi. i. fig. 1 c, cl), which may be these excretory papilla?, but here, again, their structure is not recognised. It is to Metschnikoff (31) that we are indebted for the first correct description of these structures ; he was the first to recognise their position under the velum, the opening in the apex, and their connection with the circular canal. His figures are by far the best, although fig. 2, pi. v., is open to the same objection as Haeckel's figure ; fig. 7 is, however, very plain. In M. victoria these structures seem to have a very regular arrangement, one opposite each tentacle-bulb and one in the middle of the space between consecutive tentacles. In Plate X X I I. fig. 2 is represented a section passing through the bell-margin in the plane of one of these papilla?; the opening in the apex is distinct, being connected with the circular canal by a passage lined with strong cilia. These papilla? do not seem to be present in young specimens, only the larger and older examples appear to possess them. It is doubtful if their function is purely excretory. Brandt (6, pi. v. fig. 4) shows the continuation of the radial canals into the mouth-lobes of his Mesonema {Zygodactyla) ccerulescens, similar to their continuation into the mouth-lobes of M. victoria, as already described. He also shows a peculiar rectangular communication of these canals, on the mouth-lobes, which is not present in our species. While collecting off the inlet of Victoria Harbour early in July, on several occasions lens-shaped masses were picked up, they were about 3 cm. broad; towards the end of the month, when this Medusa was obtained, they proved to be the central gastric peduncle from which the peripheral parts had been worn and macerated away, leaving a hard, smooth, lens-shaped mass. Fewkes has well described these from M. cyaneum as ". . . . flat, slightly convex above, rounded convex below." He regards them as probably homologous with the gelatinous peduncle of genera like Lirope, Geryonia, and Carmarina. M. victoria frequently contracts the bell when disturbed in a characteristic manner, the thin flexible margin being folded in, the tentacles almost rolled under the velum, the gastric peduncle making a rounded protuberance on the upper surface of the bell. Huxley (23, pi. xxxvii. fig. 11) shows this attitude in his drawing of Oceania I Although the |