PARTHENOGENESIS IN THE ANIMAL KINGDOM. 239 



Oil account of tlieir various waudcrings tlirongh dilTereiit auimal 

 bodies, the frematodcs, aud more es[)ecial!y certain species of the genus 

 (listomaj so called ou account of two suckers or stomata on the flat part 

 of their bodies, are of peculiiir 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, (Limnaius, 

 Plauorbis, &c.,) and i)enetrates 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 vshape, 

 and arc converted into small animals. These essentially possess the 

 structure of mature trematodes, but are sexless and have a tail-like ap- 

 pendage; they increase slowly in size and expand the worm which 

 contains them, and which seems to have no other function than to pro- 

 tect them and promote their development, >. e., to act as their nurse. 

 When completely developed they pierce the envelope of their nurse 

 aud 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 

 liuder the name of cercaria, Nitz ; but their relation to the trematodes 

 was unknown until quite recently. The cercaria afterward 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 evolu- 

 tion 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, (iu the liver of sheeji, cattle, 

 goats, and deer;) also in asses, hogs, hares, etc., and in rare cases in 

 man. f Distoma hcpaticum^ L. ; Distoma JtamiatoMum, Bilhars*) 



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 larva? is produced, which 

 act as nurses of the cercaria, so that the worm resulting from the em- 

 bryo might be called the grand-nurse. 



Thus the numerous and fertile multiplication of germs by means of 

 agamic reproduction counterbalances the difficulties and obstacles 

 which these animals have to encounter in their various migrations 

 through other organisms before they reach their perfect form. 



Formerly the tapeworm was considered nothing more than a simple 



* Zeitscbrift fiir wissenschaftlicbe Zoologie, 1853, vol. iv, pp. 53-70 and 454-456. 



