1888.] MICKOSCOPICAL JOURNAL. 169 



that only an experienced observer can appreciate the difference. Indeed the 

 great naturalist, Louis Agassiz, so late as 1852, in the New Haven Journal 

 of Science and Art, declared that Opalzna was the missing link in the his- 

 tory of Distonia, a genus of parasitic worms, and further, that the embryo 

 hatched from the egg of a planarian (another worm) was a genuine polygas- 

 tric animalcule of the genus Paramecium. In the same paper he says, refer- 

 ring 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 con- 

 dition of worms.' He adds, also, this: — ' Since I have ascertained that the 

 Vorticellce are true Bryozoa * * * 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 sta*tement 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 eluci- 

 dating 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 

 permanent firm axis. Their power of extension and retraction render them 

 organs of locomotion and pi'ehension. 



The corticated forms have, protruding fi-om the surface at well defined 

 regions, thread-like extensions of the protoplasm, called, if but few in num- 

 ber and relativel}^ very long, fiagella, and cilia if numerous and relatively 

 short. These, by lashing the medium, propel the animal, or, if anchored, 

 drive currents past the oral aperture ; whilst in the highest divisions the cilia 

 are replaced by stvles or setae which act very much like walking organs, and 

 in the still more highly endowed Tentaculifera the prehensile prolongations 

 of the body substance are 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 of this 

 group in which the tentacles become marvellously flexible. This is notably 

 the case in Ephelotida and in Podophrya Jlexilis, a fresh-water species de- 

 scribed by myself in The Microscope for August, 1887. In this form, the 

 long, extensible and constantly writhing arms remind one of a veritable 

 Octopus. 



Another equally instructive series is that of the manner of and contrivances 

 . ' food ingestion. In the simplest forms this takes place by simply engulf- 

 '"§ it ; a little higher in the series it is received through the body walls at re- 

 stric^jj areas ; then a well defined and guarded aperture is found, often rein- 

 lorcec i^y ^ wonderfully complex system of chitinous or otherwise indurated 

 appencj^ggg^ or it may consist of sucking tubes, sometimes flexible. But 

 enough ^^ these details, which have been enumerated not onl}- to show the 

 mutual relations of the groups which result from the fact of descent from 

 common ancestors, but to present certain terms by which to make easy the ex- 

 planation q£- ^j-jg persistency of protozoic functions in the associated cells of 

 the tissues of multicellular animals. Thus the amoeboid motion of the color- 

 less blood -orpuscles and other cells, the contractions of the muscle cells, the 

 cilia or tht, epithelium of the trachea and ventricles of the brain are 

 examples. 



i he rrotozQj^^ lowly as they are in organization, and insignificant in size, 



