Observations on the development of the medusæ / by John Reid.

  • Reid, John, 1809-1849.
Date:
[1848]
Catalogue details

Licence: Public Domain Mark

Credit: Observations on the development of the medusæ / by John Reid. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    more powerful object-glasses necessary for distinguishing these, without compressing more or less the part under examination. The internal is considerably thicker and more opake than the external layer, is of a slightly yellowish colour when it accumu- lates at any point in greater abundance than usual, and is folded inwards to form the four equidistant projections seen on the sur- face of the stomach when the mouth is dilated (fig. 5 a), and when the body of the animal is slit open and then spread out (fig. 6 c). By making a transverse section of the body, the rela- tive thickness of the internal and external layers, and the man- ner in which the internal is folded to form the four pouches or short canals that project from the internal surface, are very di- stinctly seen (fig. 7). These four short canals (fig. 7 a) termi- nate at their upper end in another canal, encircling the mouth and placed between it and the margin of the disc (fig. 6 b). Into this circular canal the hollow tentacula also open. The inner surface of the circular canal and the tentacula is lined by the in- ternal layer. The four depressions (fig. 2 a) placed between the mouth and margin of the disc correspond to the termination of the four vertical in the circular canal. Across the bottom of these depressions, which at first sight look like apertures, a mem- brane is stretched sufficiently thin to permit readily of the trans- udation of fluids. After reading Steenstrup’s observations on the structure of these animals*, where he describes four canals,—one in each angle of the extensible membrane surrounding the mouth and forming the lips,—passing from the circular canal already men- tioned, and also another circular canal placed in the free margin of the lips, I repeated my examinations; and though I used glasses of very different magnifying powers, and made numerous trials, I could not satisfy myself of the existence of these canals. No doubt four equidistant white lines presenting the appearance of canals are seen, in certain conditions of the extensible lips, running in the positions indicated by Steenstrup; but in some of the numerous forms which the lips assume these lines entirely disappear, and when present they seem to be formed by narrow ridges on the external surface, resulting from the quadrangular shape assumed by the lips. The free margin of the lips fre- quently presented indications of the presence of a canal, but I could never satisfy myself of its actual existence. In making such investigations, it must be kept in mind, that the internal is readily separated by pressure from the external layer, otherwise we may be led into error. In the almost daily examinations I have made of these animals during the last two years, I never * On the Alternations of Generations, &c., translated for the Ray So- ciety, pp. 22, 23.