236 ALTERNATE GENERATION AND 



iu the third, fourtli, or yet further removed generations. The pecu- 

 liarity of this phenomenon not only consists in the alternation of different 

 progeny, but also in that of sexual and sexless reproduction. One gen- 

 eration may consist of sexually developed males and females, and bear 

 young from eggs, and the next may be sexless, and may bring forth 

 young by fission, by buds or germs. These animals capable of agamic 

 pro])agation were called nurses by Steenstrup, because it is their function 

 to i^rovide for the alimentation and development of the sexual animals. 

 The number of sexless intermediate generations, as well as their degree 

 of development and organization, differs in different species. They 

 either jjossess provisory or temporary organs, and are therefore lar\ ae, 

 or they are fully developed individuals, and already show the construc- 

 tion and mode of life of the sexual animals. The sexless larvae of 

 animals, such as butterflies, which undergo simple metamorphosis, are 

 distinguished from our nurses by their inability to multiply by agamic 

 reproduction; vso that we may, according to Leuckart, consider alternate 

 generation with nurses as a metamorphosis combined with agamic repro- 

 duction. 



Alternate generation, very aptly called metagenesis by E. Owen, was 

 first observed in the salpse, a kind of mollusks which are as remarkable 

 for their form as for their mode of life. They belong to the tunicata, 

 and are found in great numbers in the ocean, the Mediterranean, and iu 

 all southern seas. They swim about a little below the surface, and pre- 

 sent the appearance of oval or cylindrical bodies, clear as crystal, moving 

 about either isolated or united iu long chains, by taking iu water and 

 expelling it again. 



Our German lyric poet, Chamisso, remarked, in his voyage around the 

 world, that the isolated salpaj could not be members of a severed chain, 

 because they did not resemble the individuals of the latter. He further- 

 more recognized that the solitary sal pre always contained a progeny 

 resembling the chain^ while the individuals of the latter contained a 

 foetus formed exactly like the solitary salpai. Chamisso published his 

 interesting observations in 1819, at Berlin, in an essay entitled De 

 animalibus quibusdam e classe vermium linncvana, Fasc. I. de Siilpa, in 

 which he expressed the view that the solitary salpai proceeded from the 

 individuals of the chain and the latter from the solitary ones. Cha- 

 misso's discovery was but little appreciated at first 5 it was even ridi- 

 culed as the vagary of a poet, until it was brilliantly defended by 

 Steenstrup in 1842, and confirmed and expanded later by the accurate 

 investigations of other zoologists. We know now that the loosely con- 

 nected chain is composed of hermajdirodite sexual animals, generating 

 an embryo usually from one egg only, which remains connected for a 

 time with the mother by means of a kind of placenta, and is nourished 

 by it until, having attained a considerable size, it escapes and forjus the 

 solitary or isolated salpa — the only case of viviparity among the tuni- 

 cata. The solitary salpa then generates a chtiin of sexually developed 



