496 LIFE HISTORY STUDIES OF ANIMALS. 



ephyrse, which had been found in the sea before Sars's time, and were 

 then counted as a particular kind of adult medusae. They are small, 

 flat discs with eight lobes or arms, all notched at the extremity. A 

 pile of ephyrse is produced by the transverse constriction and division 

 of the strobila in a fashion which reminds us of the rapid production of 

 the animals in a Noah's ark by the slicing of a piece of wood of suitable 

 sectional figure. It was thus ascertained that the scyphistoma, strobila, 

 and ephyra are successive stages of one animal, but for a time no one 

 could say where the scyphistoma came from, nor what the ephyra 

 turned to. At length Sars, aided by the anatomical researches of 

 Ehrenberg and Siebold, was able to clear up the whole story. The 

 ephyra is gradually converted by increase of size and change of form 

 into an Aurelia, a common jelly-fish which swarms during the summer 

 in European seas. The aurelia is of two sexes, and the eggs of the 

 female give rise to ciliated embryos, which had been seen before Sars's 

 time, but wrongly interpreted as parasites or diminutive males. These 

 ciliated embryos, called planulae, swim about for a time and then settle 

 down as polyps (scyphistomata). There is thus a stage in which 

 aurelia divides without any true reproductive process, and another 

 stage in which it produces fertile eggs. There is alternation of genera- 

 tions in aurelia as well as in salpa, and Sars was glad to fortify by a 

 fresh example the observations of Chamisso, on which doubts had been 

 cast. 



It was not long before the alternation of generations was recognized 

 in hydromedusai also, and then the ordinary hydrozoau colony was yeen 

 to consist of at least two kinds of polyps, one sexual, the other merely 

 nutrient, both being formed by the budding of a single polyp. The 

 sexual polyp, or medusa, either swims away or remains attached to the 

 colony, producing at length fertilized eggs, which yield planulse, and 

 these in turn the polyps which found new colonies. 



Those of us who are called upon to tell this story in our regular course 

 of teaching should not forget to produce our scyphistoma, strobila, and 

 ephyra. The interest is greatly enhanced it they are shown alive. It is 

 not hard to maintain a flourishing marine aquarium, even in an inland 

 town, and a scyphistoma may be kept alive in an aquarium for years, 

 budding out its strobila every spring. 



Alternation of generations when first announced was taken to be a 

 thing mysterious and unique. Chamisso brought in the name, and 

 explained that he meant by it a metamorphosis accomplished by suc- 

 cessive generations, the form of the animal changing not in the course 

 of an individual life, but from generation to generation (forma per gen- 

 erationes, nequaquam in prole seu individuo, mutata). Sars adopted 

 Chamisso's name and definition. Steenstrup a little later collected and 

 discussed all the examples which he could discover, throwing in a 

 number which have had to be removed again as not fairly comparable 

 with the life histories of Salpa and Aurelia. He emphasized the alter- 



