304 ■ The Microscope. 



the parent, but a ciliate type, the planula, so closely resembling the 

 parasite from the frog that only an experienced observer can appre- 

 ciate the difference. Indeed, the great naturalist, Louis Agassiz, so 

 late as 1852, in the New Haven Journal of Science and Art, 

 declared that ojialina was the missing link in the history of distoma, 

 a genus of parasitic Avorms, and, further, that the embryo hatched 

 from the egg of a planarian (another worm) was a genuine, poly- 

 gastric animalcule of the genus paramecium. In the same paper he 

 says, referring to the above, "with such facts before us, there is no 

 longer any doubt respecting the character of the polygastrica; they 

 are the earliest larval condition of worms." He adds also this: 

 " Since I have ascertained that the vorticelke are true bryoza * * * 

 there is not a type of these microscopical beings left which hereafter 

 can be considered a class by itself in the animal kingdom." These 

 sentences are not quoted to call attention to an error of our revered 

 naturalist, but to show, more thoroughly than a mere statement 

 would do, the absolute similarity of the ciliate embryos of certain 

 metazoa to ciliate infusoria. 



The study of protozoa in the light of the above and for the 

 sake of elucidating such questions of world-wide interest cannot be 

 lightly esteemed. 



The simplest rhizopoda, as stated above, consist of naked 

 reticulated protoplasm; from this unmodified beginning may be 

 traced ever increasing complexity of structure. The locomotory 

 organs may serve for an example. The uncovered forms move in 

 two ways, by a flowing or streaming of the protoplasm as a whole, 

 or by the protrusion of finger-like processes or threads of the body 

 substance, called pseudopodia, which are transient or held by a per- 

 manent firm axis. Their power of extension and retraction render 

 them organs of locomotion and prehension. 



The corticated forms have, protruding from the surface at well 

 defined regions, thread-like extensions of the protoplasm called, if 

 but few in number and relatively very long, flagella, and cilia if 

 numerous and relatively short. These, by lashing the medium, 

 propel the animal, or, if anchored, drive currents past the aperture; 

 whilst in the highest divisions the cilia are replaced by styles or 

 setcTe which act very much like walking organs, and in the still more 

 highly endowed tentaculifera the prehensile prolongation of the 

 body substance is tubular; usually with sucking disc at the 

 extremity, and often with a spiral coiled fibre for its retraction. 



A still more highly specialized instance occurs in certain ones 



