626 SUMMARY OF CURRENT RESEARCHES RELATING TO 



instead of becoming a hydra, becomes a degraded actinula, a mouth- 

 loss, untentaculated "root"; this gives rise to hydro, which in turn 

 produce medusa buds. In this form a secondary alternation is thus 

 inserted in the life-history. In Hydractinia, owing to polymorphism, 

 and to a much greater extent Podocoryne, a still more complicated 

 history is gone through. The " root " buds off nutritive hydra?, each of 

 which buds off three sorts of polyps, one of which, the blastostyle, 

 buds off medusa), which again bud off other medusa?, which produce 

 eggs. Here several secondary alternations are intercalated. The case 

 of Hydractinia is regarded as beginning to simplify its life-history by 

 the degradation of the sexual medusa? into sessile reproductive 

 organs. 



The author's theory is that the remote ancestor of the Hydro- 

 medusa? was a solitary actinula with no medusa stage, but probably 

 the power of budding. This actinula became more and more adapted 

 to swimming until it became converted into a medusa, developing 

 straight from the egg without alternation of generations. Having 

 reached this stage, the larva acquired the property of fixing itself, 

 and then multiplied by budding off similar larva?, which became 

 medusa?. This fixed condition having become perpetuated by natural 

 selection, the primary larva ceased to become a medusa, but remained 

 a sessile larva and budded off larva? which became sexual medusa?. 

 The medusa characteristics of these secondary larva? became accele- 

 rated, and the primary larva acquired the power to produce larva? 

 which like itself remained sessile. In this way sessile hydra com- 

 munities with medusa buds and free sexual medusa? were evolved ; 

 finally these became polymorphic ; and gradually the free medusa? were 

 degraded to medusa-buds or sexual buds on the bodies of the sessile 

 hydras. 



Nematocysts in the Siphonophora.*— M. M. Bedot finds in the 

 Velellida? two sorts of cnidoblasts, provided with stalks (" tiges "), 

 the nematocysts of which are distinguished by the presence or absence 

 of a barb at the base of the thread, as well as by their difference in 

 size. In the large cnidoblasts, muscular striations are seen at the 

 base of the stalk ; this also shows at its terminal region a spindle- 

 shaped organ, which encloses a spirally coiled filament and a highly 

 refracting spherical body. The cnidoblasts which are scattered 

 through the ectoderm of the tentacles in the Velellida? and which are 

 not grouped to form batteries are deprived of stalks and cnidocils. 



The Physalida? also present two forms of nematocysts, neither of 

 which are barbed. On the fishing filaments there is every stage 

 between cnidoblasts with and without a stalk. From this one is led 

 to conclude that the stalk is formed from the cnidoblast itself, and not 

 from a neighbouring cell, as was supposed. The nematocyst arises in 

 the interior of a small spherical cavity, which has become formed in 

 the cnidoblast, and which is filled with a transparent fluid. From the 

 wall of this cavity a small bud or " nematoblast " arises which gradually 

 projects into the fluid. It increases greatly and ultimately nearly 



• Arch. Sci. Phys. et Nat., xv. (18SG) pp. 415-6. 



