676 REPORT— 1897. 



Bergen, as parish priest, and betook himself to the lifelong study of the animals of 

 the Norwegian seas. He soon found out that his Scyphistoma was merely an 

 earlier stage of his Strobila. Scyphistoma has a Hydra-like body, less than half 

 an inch long, and drawn out into a great number of immensely long tentacles. 

 It buds laterally like a Hydra, sending out stolons or runners, which bear new- 

 polyps, and separate before long, the polyps becoming independent animals. In 

 the midst of the tentacles of the scyphistoma is a prominence which bears the 

 mouth. This grows upwards into a tall column, the strobila, which is supported 

 below by the scyphistoma. When the strobila is well nourished it divides into 

 transverse slices, which at length detach themselves, and swim away.' These are 

 the Ephyrae, which had been found in the sea before Sars' 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 ephyrae is produced 

 by the transverse constriction and division of the strobila m 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' time, but wrongly interpreted as parasites or diminutive males. These 

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

 polyps (scyphistomata). There is thus a stage in which Aurelia divides without 

 any true reproductive process, and another stage in which it produces fertile eggsi. 

 There is alternation of generations 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 recognised in Hydro- 

 medusae also, and then the ordinary Hydrozoan colony was seen 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 awaj 

 or remains attached to the colony, producing at length fertilised eggs, which yield 

 planulae, 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 gi-eatly enhanced if 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 successive generations, the form of 

 the animal changing not in the course of an individual life, but from generation to 

 generation (fonna per generationes, neqiutquam in prole seu indwiduo, midata). 

 ■ Sars adopted Gbamisso'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 emphasised the alternation of budding with 

 egg-production, and the unlikeness in form of the asexual and sexual stages Like 

 Chamisso, he carefully distingiushed between development with metamorphosis 



» 'Lencks.rt {ZeiU. f. wiss. Zool., Bd. III. p. 181) remarks that elongate animals 

 tend to divide transversely or to bud axially, while broad animals tend to divide 

 longitudinally or to bud laterally. The question has been raised more than once 

 whether the division of the strobila is not really a case of budding. Leuckart shows 

 that budding and fission cannot be separated by any deOnition ; they pass insensibly 

 into one another. ( Waffner's Handh. d. Physiol., art. ' Zeugung.') 



