240 ALTERNATE GENERATION AND 



animal having a liead and an articulated body. Since Steenstrup's 

 time, however, and especially throngli the more recent investigations of 

 V. Siebold and van Beneden, we know it to consist of a chain or colony 

 of differently-formed individuals. The larger posterior joints (the so- 

 called profjlottides) represent the organs of generation, and contain 

 many thousand eggs in their ramified ovaries. In these, microscopic 

 embryos are developed, which are discharged when the ripe joints tall 

 off with the excrement of the host. The embryos do not then leave the 

 eggs at once, but remain in their envelopes, which are well fitted for re- 

 sisting putrefaction or chemical agents, until the eggs are accidentally 

 swallowed by some (usually an herbivorous) animal. In the intestines 

 of the latter the embryo forces its way through the egg-envelope, 

 softened by the digestive juices, pierces the intestinal walls and neigh- 

 boring tissues, until it reaches a vein and is carried by the blood to 

 more distant organs, in whose parenchyma it remains. After losing its 

 embryonic hooks, the tape-worm larva growls to a bladder of varying 

 size, along the walls of which numerous buds (the later " heads") arise 

 in such a manner that the hollow body of the tape-worm head extends 

 into the bladder. Such colonies were long known and considered aS 

 different species of animals. When one of them gets into the intestines 

 of a larger animal, the head or bud provided with hooks and suckers 

 is turned inside out, the bladder is digested, and the joints of the tape- 

 worm (the real sexual, hermaphrodite individual) begin to grow behind 

 the head. The reproduction of the tape- worm, therefore, passes through 

 three different phases : The worm -like embryo or grand-nurse, the so- 

 called tape- worm head or nurse, and the mature sexual animal. 



With the exception of the salpse, we have so far only considered cases 

 of metagenesis where the nurses are in the form of larvse. In the arthro- 

 2)ods, among the dqjfera, we also find such nursing larvre — an entirely 

 new and remarkable phenomenon first discovered in the fall of 1801 by 

 Nicholas Wagner, professor of zoology, in Kasan. It produced no small 

 excitement among zoologists, and was the cause of so much astonishment 

 that V. Siebold himself designated it as hardly credible on receiving, 

 after some delay, Wagner's essay in the " Zeitschrift tiir wissenschaftliche 

 Zoologie," 18G3, vol. xiii, p. 513. Wagner could not then describe 

 clearly tlie insect-larva which he had recognized as capable of reproduc- 

 tion, and v. Siebold took it from the illustrations to be a cecydomyde 

 larva. Not long after, however, Dr. F. Meinert,* of Copenhagen, not 

 only fully confirmed his beautiful discovery, but extended it by proving 

 the different phases of development up to the imago, which Wagner t 

 had meanwhile also accurately investigated. Meinert calls it the mias- 

 tor metraloas, but according to the later researches of our excellent 

 dipterologist. Dr. Schiner, reported to the imperial zoological-botanical 



* Zeitsclirift fur wissenscbaftliche Zoologie, vol. xiv, p. 394. 

 t Vol. XV, p. 106. ■ ' ^ 



