There are many modifications observed in the different genera as regards the number of tentacles, the number and character of the radial canals, the minute structure of the sense-organs, and some other characters, but they agree in having a velum, ocellar sense-organs, and manubrial sexual organs. The tentacles are rudimentary in _Amalthea_; in _Corymorpha_ there is only one tentacle; in _Perigonimus_ there are two; and in _Bougainvillia_ they are numerous; but the usual number is four or six. The radial canals are usually simple and four in number, but there are six in _Lar sabellarum_, which branch twice or three times before reaching the margin of the umbrella (Fig. 132, B). [Illustration: FIG. 131.—Medusa of _Cladonema_, from the Bahamas, showing peculiar tentacular processes on the tentacles, the ocelli at the base of the tentacles, the swellings on the manubrium that mark the position of the gonads, and the radial and ring-canals of the umbrella. (After Perkins.)] There can be no doubt that the Medusae of many Gymnoblastea undergo several important changes in their anatomical features during the period of the ripening of the sexual cells. Thus in _Lar sabellarum_ the six radial canals are simple in the first stage of development (A); but in the second stage (B) each radial canal bifurcates before reaching the margin, and in the adult stage shows a double bifurcation. The life-history has, however, been worked out in very few of the Anthomedusae, and there can be little doubt that as our knowledge grows several forms which are now known as distinct species {267}will be found to be different stages of growth of the same species. [Illustration: FIG. 132.—Two stages in the development of the Medusa of _Lar sabellarum_ (_Willsia stellata_). A, first stage with six canals without branches; B, third stage with six canals each with two lateral branches. The developing gonads may be seen on the manubrium in A. (After Browne.)]