OCR Text |
Show 1897.] MR. E. T. BHOWNE ON BRITISH MEDUSA. 831 the younger ones vary between 114 and 180. In the smallest specimen the generative cells are just beginning to make their appearance along some of the radial canals. The generative cells are situated in the wall of the canal, and as they develop in size the wall of the canal extends downwards, so that in the largest specimens the wall hangs down as a large fold, about 4 m m . wide, and of a purplish colour. The generative cells extend from the stomach nearly to the ring-canal, terminating in the largest specimens about 2 to 3 m m . from the margin. In this short portion the canal is very broad, several times broader than the parts bearing the gonads. Haeckel states that Polycanna funglna has alternating sterile and fertile radial canals. The Valencia specimens have generative cells on almost every canal, occasionally a sterile canal occurs in one or two places on the umbrella. As the canals are of different ages of growth, so are the generative cells, and consequently the generative folds vary in size. Strethill Wright was successful in rearing the ova of Polycanna vitrina (Gosse), and showed that au alternation of generations existed, but the hydroids died at too early a stage for a satisfactory determination of their generic position. The Tentacles and Marginal Vesicles. The smallest specimen shows that the medusa probably starts its free-swimming life with four tentacles, and later on has eight and then sixteen, which correspond to the perradial, interradial, and adradial sets, as in Phialidium. After sixteen are present the tentacles develop in an irregular manner until the maximum number, which is not yet known, has been reached. The largest of the Valencia specimens has 56 tentacles and about 150 marginal bulbs; if the latter develop tentacles, a full-grown specimen would have about 200 tentacles. The first appearance of a tentacle is marked by a very small bulb upon the margin of the umbrella, and the bulb grows to a considerable size before the tentacle begins to grow out from it. The bulbs increase in number as the umbrella increases in size, and consequently, in the later stages, the bulbs and tentacles show various stages in development. A bulb carrying a full-grown tentacle is very large and marked externally with four or more longitudinal ridges. As the wall of the bulb is very thin, there is a large internal cavity which communicates with the ring-canal through a narrow opening. The bulbs and the tentacles are of a light purplish colour. The arrangement of the tentacles on the margin of the umbrella does not coincide with the arrangement of the radial canals. The marginal vesicles are exceedingly numerous aud irregular in position ; there are usually two vesicles between every two marginal bulbs, and the number varies between one and three. Each vesicle usually contains four otoliths, but the number varies from two to 'six. The arrangement of the otoliths inside the |