88 LECTURE V. 



viz., that tliey were spontaneously genei'ated. No other exphxnation 

 in the then state of the knowledge of development of the Entozoa 

 appeared to be adequate to account for the fact of their getting into 

 the interior cavities and tissues of higher animals. Subsequent 

 researches have, how^ever, shown that the tapeworm quits the 

 ovum as a minute locomotive Echinococcus, and exchanges its 

 6-uncinated character for a head like that of the Tcenice arinatce, in 

 an encysted pupal state, within the body of some animal which 

 is the natural food of that higher species, in which the ultimate 

 development of the tapeworm is to be effected. The chances 

 against the introduction of such a minute ovum or embryo ai"e, of 

 course, great, but these impediments are met by the incredible 

 numbers that are developed in a single individual of the Tceiiia or 

 Bothriocephalus. The mode of introduction of the Entozoa of the 

 order Trematoda becomes in like manner more intelligible as the 

 phenomena of their development are better understood. Certain 

 fresh-water snails ai*e infested by this order, as the Limncea 

 stagnalis, e. g. by the Distoma tarda. The ova, or products of the 

 ova, of this species are found in early summer, adhering in vast 

 numbers to the inner surface of the respiratory cavity, and to the 

 exterior of the lobes of the liver and generative organs of the snail ; 

 where they increase in size, and detach themselves as free animal- 

 cules, having a twisting vermicular motion, and assuming a bright 

 yellow colour, whence they were called by Bojanus " konigsgelben 

 Wurmern." If one of these be microscopically examined, none of 

 the lineaments of the organs of the future Distoma can be discerned; 

 they resemble in structure rather the Gregarin^e, consisting, in fact, 

 of little else than the cell-progeny of the primary germ- vesicle. Few 

 of the cells have perished as such, or have been metamorphosed, save 

 those that have gone to form the outer contractile skin, whilst still 

 fewer have been liquefied and absorbed into a larger subcentral cell. 

 As the growth of this Gregariniform parasite proceeds, a progeny 

 is seen to I'ise in its interior by the development of several of the 

 contained germ-cells into embryos ; these gradually acquire a ce- 

 phalic spiculum and a caudal appendage ; they escape from the 

 parent cyst and from the snail, and disperse themselves as fi'ee 

 swimming ciliated cercariform animalcules in the water. After a 

 brief enjoyment of this free and active state of existence, they shrink 

 in size, the vibi'atile tail is cast off, and they attach themselves to 

 the skin of the snail. Here they become buried, form for themselves 

 a pupa-case out of the condensed mucus, and are metamorphosed 

 into true Distomata, which gain their pai*asitic habitat by piercing 

 the soft integument of the water-snail. Thus we have a Trematode 

 cntozoon, successively assuming the form of a Grcgarina, Vi, CercariOy 



