OCR Text |
Show 1896.1 BRITISH HYDROIDS AND MEDUSAE. 465 elongate and are soon seen to be dilated into bulb-like expansions their extremities. The bulbs increase in size and come in contact by their sides; while one of them, enlarging much more rapidly than the other three, gives a marked preponderance to its side of the bud and makes the distal end of the bud appear obliquely truncated. It then begins to extend itself beyond this distal end into a thick hollow tentacle." Allman also obtaiued free-swimming medusae from his hydroids, which he has described and figured in his monograph. These are similar to the figures given by Hodge, and have a margin at right angles to the longitudinal axis of the umbrella. From the description given by Allman of the development of the medusa, it appears that the rapid growth of the large tentacle-bulb extends the length of the umbrella more on that side than on the other, and in this way the margin becomes oblique, as one side of the umbrella is longer than the other. It is clear from the descriptions given by Hodge and Allman, that the obliqueness must disappear before the medusa is liberated, as they figure the free-swimming form with the margin in the normal position. I think that these observations on the obliqueness of the margin of the umbrella strengthen the view that the British and Norwegian specimens belong to the same species, and I have again united them under the old name of Corymorpha nutans. Forbes has given only two species in his medusoid genus Steenstrupia, namely S. rubra and S. fiaveola. I agree with Haeckel in considering that these are not distinct species. The characteristic features of these medusae are similar to those of the medusae of Corymorpha nutans. The figures given by Forbes of Steenstrupia show the characteristic points of the species, but they are not good figures of a healthy specimen of the medusa of Corymorpha. Hodge recognized the similarity between Steenstrupia and the medusa of Corymorpha, but was led by Forbes's figures to consider the latter as another species. Allman states the medusa of Corymorpha " belongs to a form to which Forbes has given the generic name of Steenstrupia" Sars (1877) writes, " It is even not improbable that when disconnected from the parent animal, they (the medusae) may develop themselves into the species described by Forbes under the name of Steenstrupia rubra." During m y visit to Valencia Island in 1895, I found medusae corresponding to the descriptions and figures given by Hodge and Allman of the medusa of Corymorpha nutans (PI. X V I . fig. 1). They were often exceedingly abundant during April and May, and some occasions many hundreds could have been quickly collected. One day I thought that a lovely Siphonophore had entered the net, but closer examination showed some dozens of these medusae, caught by the tentacle, on a piece of cotton about an inch and a half long. The umbrella is bell-shaped, nearly twice as long as wide; the aboral end of the umbrella is extended into a spine-like process, which is always present, and is a characteristic feature of the PROC. ZOOL. Soc-1896, N o . X X X . 30 |