OCR Text |
Show 1897.] MR. E. T. BROWNE ON BRITISH MEDUS.E. 825 from generative cells. The gonads in all the specimens were quite colourless. The margin of the umbrella is thickly beset with long tentacles, the number varying according to the size of the umbrella, as shown by the following examples :- 3 mm. diameter .... About 30 tentacles. 6 mm. „ „ 60 9 mm. „ .... „ 100 11 mm. „ .... „ 120 17 mm. „ „ 230 26 mm. „ „ 300 „ or more. The tentacles first make their appearance as bulbs on the inner side of the margin, which is fairly broad. The bulb apparently grows outwards across the margin, and the tentacles develop from the outer end of the bulb. The tentacle gradually tapers from the bulb ; it is at least 25 m m . in length, when expanded, and covered with nematocysts. The attachment of the tentacle to the umbrella is further strengthened by an outgrowth which extends from the base of tbe tentacle for a short distance along the ex-umbrella. The outgrowth is usually triangular in shape and consists of large cells. This outgrowth or " spur " appears after the development of the tentacle, and is absent or scarcely visible in young stages. W h e n the medusa is motionless in an aquarium, the tentacles are arranged in two positions. The basal portion of one teutacle points upwards, and that of the adjacent one slopes slightly downwards ; the basal portion is stiff and straight, the outer portion gracefully curves over, with the free end hanging down and tapering out to an almost invisible thread. O n the inner side of the basal bulbs of the tentacles, and just below the ring-canal, there is usually situated a black ocellus. The ocelli show a considerable amount of variation in number. As a rule there is not one opposite every tentacle, but every alternate tentacle. In the same individual several adjacent tentacles may each have an ocellus at their base, then an alternating series, one with, and the next without an ocellus ; aud followed by two or three adjacent tentacles without ocelli. In one large specimen I counted 228 tentacles, but only 88 ocelli, and in another large specimen I noticed an ocellus opposite nearly every tentacle. O n the inner side of the margin of the umbrella are situated the curious sensory clubs or cordyli-one about midway beween every two tentacles ; they usually point inwards, and lie close to aud below the velum. Brooks has recently given a description and figures of these organs. In addition to the tentacles and cordyli there are also long filiform cirri, armed with nematocysts : usually one between every two tentacles, but often two are present, and are situated close to the bulb of the tentacle. In a few specimens I noticed that a PROC ZOOL. Soc-1897, No. LV. 55 |