938 SPECIAL PHYSIOLOGY. 



believed respectively to represent the male and female products, not 

 germ- and sperm-cells, but germ- and sperm-nuclei. 



This mode of reproduction, by alternate generation, often presents 

 examples of genetically related animal forms, exhibiting not only a non- 

 sexual character, but a totally different shape and organization, as 

 compared with the parents. The ovum of certain Echinodermata, of 

 the Echinida and Ophiurida for example, develops into a free-swim- 

 ming ciliated embryo, which becomes converted into a medusa-like 

 larva, known as the pluteus, a form which has quite a Coelenterate 

 type ; but in the body of this, near the digestive cavity, close upon the 

 remaining substance of the original ovum, or yolk-mass, a young Echi- 

 nus appears in the form of a circular disc, which gradually assumes a 

 quinary radiated form, and ultimately becomes a perfect Echinoderrn. 

 In the same way, the ova of the Taeniae, or Tape-worms, taken by ani- 

 mals which live upon offal, or swallowed by Man in water, pass into 

 the alimentary canal, and there develop into Echinococci, or Cyaticerci, 

 which penetrate, whilst very minute, the surrounding tissues by a pro- 

 cess of boring, and so find their way into all parts of the body, and 

 there grow as Cysticerci or Echinococci. The tissues of the edible 

 animal (as a pig, for example), thus infested, being then eaten, the 

 Echinococci, if not destroyed by the cooking, attach themselves to the 

 intestinal mucous membrane of the person who eats them, and form 

 the head of a tcenia, which then, by successive fission, produces its 

 long segmented body, each section of which, now named a proglottis, 

 is really independent of the rest, and is provided with true reproduc- 

 tive organs, sperm-cells and ova. The Trichina is not, as was once 

 supposed, an intermediate form, by alternate generation. 



Lastly, in the interior of certain Trematode worms, such as the 

 Pi an arise 1 , and Distomata, a succession of non-sexual larvae is devel- 

 oped, each producing others within them, until, at last, sexual forms 

 appear resembling the original parent. Thus a Distoma, for example, 

 which is found as an entozoary parasite in the Limnseus, a fresh- 

 water snail, develops ova, which are evolved into elongated larvae of 

 very simple organization; these larvae are composed simply of nucle- 

 ated cells, which grow into ciliated organisms, and then burst through 

 the skin of the larva, attach themselves to a Limnaeus, and, having be- 

 come metamorphosed into a true Distoma, perforate the tissues of the 

 snail. 



In these cases of alternate generation, there occurs, therefore, a 

 sort of metamorphosis, because the cycle of evolution is at last always 

 completed, by a return to the parent form ; but the stages of the met- 

 amorphosis supervene in different generations, and not in the same 

 individual. So likewise, in all cases of non-sexual reproduction, 

 whether by so-called fission, by gemmation, external or internal, or 

 by recognized pseudova, a return, at last, takes place to sexual de- 

 velopment, by true. ova which require fertilization. Hence the latter 

 mode of reproduction appears the more important function, to which 

 is assigned the continuance of the specific forms of animal life. 



In the Molluscoid Tunicata, however, and in Insects, Crustacea, 

 and certain Fishes, and also in Amphibia, a true metamorphosis occurs 



