| OCR Text |
Show 1 896.1 BRITISH HYDROIDS AND MEDUSiE. 469 polypites. The reproductive buds are borne in clusters of three or four on the upper portion of the body, and when matured detach themselves as free medusiform zooids (planoblasts); they are destitute of an ectothecal covering, and are therefore freely exposed to the surrounding water. In an early stage of development the buds are much elongated, and take on their hemispherical form as they approach maturity. The planoblast, at the time of its liberation, is almost hemispherical in form ; the umbrella is perfctly colourless and destitute of thread-cells. The digestive sac or manubrium is very mutable in shape; normally it is subcylindrical, and somewhat swollen at the base, with a slightly lobate mouth. Six radiating canals traverse the umbrella, terminating on the margin in as many oval bulbs of a brownish colour, from which six smooth tentacles originate. Both ocelli and lithocysts are wanting; but halfway between every two tentacles a minute sac occurs on the margin of the umbrella, containing two or three glittering bodies, which appear to be thread-cells. The planoblast, when detached, bears'with it a portion of the peduncle which had formed the bond of connexion between it and the parent stock ; this survives as a somewhat conical process above the base of the manubrium, but it is no doubt absorbed after a time. Six is an unusual number for the radiating canals ; amongst the British Hydroida it is met wdth only in Clavatella (which has also occasionally four) and in the genus Willsia of Forbes." I have every reason for believing that the young medusa of Lar is the first stage in the development of the medusa which has been described by Forbes as Willsia stellata. At Plymouth in September 1893 the medusoid Willia stellata was fairly abundant, and sufficient specimens were obtained to trace its connexion with a medusa which exactly corresponds to the description and figures given by Hincks of the medusa of Lar sabellarum. During April and May, 1895, I again met wdth Willia stellata in Valencia Harbour, Irelaud, and was able to confirm the observations made at Plymouth. Forbes first discovered Willsia stellata at Oban in 1845, and dedicated the genus to Dr. Will of Erlangen. Agassiz changed the spelling to Willia. Porbes has described the adult form of medusa. The early forms of it may be naturally divided into three distinct stages, according to the number of tentacles :- First Stage. Six uniform tentacles. (Plate X V I . fig. 3 . ) - Umbrella, about one millimetre in length and width, varying slightly in shape but usually subhemispherical, with a broad velum. The manubrium reaches about halfway down the umbrella-cavity. In some specimens taken at Valencia the apical stalk (" Stielcanal" of Haeckel), which connected the medusa to the hydroid, was still visible in the mesoglcea of the umbrella, running from the base of the stomach nearly to the ex-umbrella. In most specimens the apical stalk had disappeared. |
