OCR Text |
Show 1896.] BRITISH HYDROIDS AND MEDUSA. 471 measuring in length the growth of the umbrella. Each series arises either on or very close to the margin, and as the umbrella grows in length so does the distance increase between the margin and each series. If all the clusters were present, they wrould form, at the adult stage, four rows, one above the other-the uppermost set, with six clusters, representing the first stage, and the lowest, with twenty-four, belonging to the adult stage. Forbes does not mention the clusters of nematocysts in his description of the species. Gosse has again described the adult, and states that five or six clusters are usually present. H e gives a figure of a cluster, and of a nematocyst which has discharged its thread. Adult Stage. Twenty-four uniform tentacles.-The addition of a fourth set of six tentacles brings the young medusa to its adult stage. The new tentacles develop just like the previous ones, and each one occupies a position midway between the tentacles belonging to the first and third stages. I have only seen one specimen which shows the connexion between the third stage and the adult form. In this specimen the third branch does not leave the main radial canal like the first two branches, but is given off from the second branch (the branch developed in the third stage) and runs down between the main radial canal and its second branch to the ring-canal, opposite the new bulb. The subsequent growth of the umbrella so separates the branches as to give the appearance of each canal dividing, about the middle of its course, into four branches, each branch running to one of the tentacles placed about equal distances apart on the margin of the umbrella. The characteristic features of the different stages may be stated thus:- First Stage.-6 tentacles ; 6 unbranched radial canals; 6 clusters of nematocysts. Second Stage.-12 tentacles; 1 branch to each canal; 12 clusters of nematocysts. Third Stage.-18 tentacles; 2 opposite branches to each canal; 18 clusters of nematocysts. Adult.-24 tentacles ; 3 branches to each canal; 24 clusters of nematocysts. Each set of tentacles usually appears about the same time and some grow faster than others. Forbes states that some of his specimens possessed only twenty tentacles. Abnormal Forms.-First Stage. One specimen with seven radial canals, tentacles and clusters of nematocysts. Valencia, 1895. Second Stage. One specimen with seven radial canals, each with one branch; fourteen tentacles and fourteen clusters of nemato-cysts.^ Plymouth, 1893. It is clear beyond all doubt that the hydroid Lar sabellarum |