1873.] 



THE AMERICAN BEE JOURNAL. 



195 



In a few other species, the physalidx and vellelidse, 

 the sexual animals separate from their nursing- 

 stalk and have a short, independent existence like 

 the medusa. 



The alternate generation of some of the intestinal 

 worms is attended by the most wonderful and 

 extraordinary circumstances. The most curious 

 ■opinions have prevailed until very lately about their 

 origin and reproduction. 



On account of their various wanderings through 

 different animal bodies, the trematodes, and more 

 especially certain species of the genus distoma, so 

 called on account of two suckers or stomata on the 

 flat part of their bodies, are of peculiar interest. 

 From the egg of the distoma a ciliated embryo, 

 resembling infusoria, is produced, which swims 

 about in the water, attaches itself to certain sweet- 

 water snails. (Linnaeus, Planorbis, &c.,) and pene- 

 trates into their bodies. There it grows, loses its 

 cilia, and develops a mouth and an alimentary 

 tube. Its contents aggregate into cellular heaps, 

 which gradually assume a definite shape, and are 

 converted into small animals. These essentially 

 possess the structure of mature trematodes, but are 

 sexless and have a tail-like appendage; they in- 

 crease slowly in size and expand the worm which 

 contains them, and which seems to have no other 

 function than to protect them and promote their 

 development, i. e., to act as their nurse. When 

 completely developed they pierce the envelope of 

 their nurse and move about freely in the body of 

 the snail until they pass through this also, and 

 glide through the water with a winding motion by 

 means of their tail. In this form they had long 

 been known to naturalists under the name of cer- 

 caria, Nitz ; but their relation to the trematodes was 

 unknown until quite recently. The cercaria after- 

 ward seeks a new host among the many inhabitants 

 of the water, (fish, mollusks, crabs, insect-larvae, 

 etc.,) penetrates them by means of its proboscis, 

 and there loses both its tail and the sting of its 

 proboscis, as no longer necessary to its new mode 

 of living. It is now converted into a distoma. 



If the animal finds all the conditions necessary to 

 its perfect evolution in its new host, it continues to 

 grow until it has attained maturity. If this is not 

 the case, it remains small and sexless, surrounds 

 itself with a transparent shell, which it secretes 

 from the surface of its own body, and remains in a 

 state of rest and inactivity like a pupa until its host 

 is eaten up by a larger and stronger animal. Hence 

 we find it in the intestines, the gall-bladder, the 

 biliary ducts, the kidneys, etc., of higher animals, 

 especially of ruminants, (in the liver of sheep, 

 cattle, goats, and deer;) also in asses, hogs, hares, 

 etc., and in rare cases in man. (Distoma hepaticum, 

 L. ; Distoma hsematobium, Bilharz.*) 



Sometimes it happens that the progeny of the 

 worm-like nurse does not immediately assume the 

 form of the cercaria, but that of the mother. In 

 that case an intermediate generation of larvae is 

 produced, which act as nurses of the cercaria, so 

 that the worm resulting from the embryo might be 

 called the grand-nurse. 



Thus the numerous and fertile multiplication of 

 germs by means of agamic reproduction counter- 



*Zeitschrift fur wissenschaftliche Zoologie, 1853, vol. iv, pp. 

 53-76 and 454-456. 



balances the difficulties and obstacles which these 

 animals have to encounter in their various migra- 

 tions through other organisms before they reach 

 their perfect form. 



Formerly the tape-worm was considered nothing 

 more than a simple animal having a head and an 

 articulated body. Since Steenstrup's time, how- 

 ever, and especially through the more recent inves- 

 tigations 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 proglottides) represent the organs of 

 generation, and contain many thousand eggs in 

 their ramified ovaries. In these, microscopic em- 

 bryos are developed, which are discharged when 

 the ripe joints fall 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 resisting putrefaction or chemical agents, 

 until the eggs are accidentally swallowed by some 

 (usually an herbiverous) 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 neighboring tis- 

 sues, 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 grows 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 blad- 

 der. Such colonies were long known and con- 

 sidered 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, her- 

 maphrodite individual) .begin to grow behind the 

 head. The reproduction of the tape-worm, there- 

 fore, 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 salpae, we have so far 

 only considered cases of metagenesis where the 

 nurses are in the form of larvae. In the arthropods, 

 among the diptera, we also find such nursing larvae — 

 an entirely new and remarkable phenomenon first 

 discovered in the fall of 1861 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 him- 

 self designated it as hardly credible on receiving, 

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

 fur wissenschaftliche Zoologie," 1863, vol. xiii, p. 

 513. Wagner could not then describe clearly the 

 insect-larva which he had recognized as capable of 

 reproduction, and v. Siebold took it from the illus- 

 trations to be a cecydomyde larva. Not long after, 

 however, Dr. F. Meinert,* of Copenhagen, not only 

 fully confirmed his beautiful discovery, but ex- 

 tended it by proving the different phases of devel- 

 opment up to the imago, which Wagner f had 

 meanwhile also accurately investigated. Meinert 

 calls it the miastor metraloas, but according to the 

 later researches of our excellent dipterologist, Dr. 



* Zeitschrift fur wissenschaftliche Zoologie, vol. xiv, p. 394> 

 f Vol. xv, p. 106. 



