The Microscope. 303 



the origin of the higher groups with the lower. We should also note, 

 often with astonishment, the remarkable capability of the disassoci- 

 ated, specific cell, and, by the proper comparisons, find at every 

 stage, that the same functions or attributes persist in the associated 

 units of animal tissues. 



The protozoa are separated into two grand divisions, rhizopoda 

 and infusoria. The simplest of the former are naked, possibly 

 reticulated protoplasm only, nucleated, and usually with a pulsating 

 vacuole; they lack all specialized organs of locomotion, prehension 

 or digestion, whilst the most highly specialized infusoria have thin 

 protoplasm surrounded by a firm, protecting and bounding wall, 

 well defined, and often complexly differentiated apertures for the 

 reception of food; their bodies have definite shape and their organs 

 of locomotion are well developed. But from the lowest to the high- 

 est may be traced such plain, biogentic relations that the develop- 

 ment of the highest from lower is unmistakably revealed. Regard, 

 for an example, the sedentary tentaculifera, the most highly devel- 

 oped of the infusoria; they give birth to ciliated, free-swimming 

 embryos, resembling closely the adults of one of the three classes of 

 the ciliata which ai'e less highly organized. This peculiar character- 

 istic in the embryology of the tentaculifera seems to conclusively 

 demonstrate their higher rank compared with the ciliata. On the 

 other hand, the adults are, without doubt, allied to the metazoic 

 hydrozoa, which, also, have cilitated embryos attesting their ascent 

 from the ciliata through the tentaculifera. So not only do the 

 structural peculiarities and developmental phenomena of the uni- 

 cellular animals plainly teach derivation by bigenetic descent 

 throughout the branch, but also indicate the starting point of various 

 types of the metazoa. In substantiation of this proposition, it may 

 not be amiss to point out examples in proof. Since the succession 

 of embryonic characters of the higher species appears to trace, more 

 or less certainly, the ancestral or developmental history of that spe- 

 cies, the connecting stages of the two branches of animals are, in 

 many cases, already established. The larvae of the star-fishes and 

 sea-urchins are free-swimming, little bodies surrounded by bands of 

 cilia which unmistakably disclose the ancestral afiinities of the echi- 

 noderms with the Peritrichons ciliata, the class of infusoria to which 

 the well-known vorticellae or bell-animalcules belong. Another illus- 

 tration may be mentioned. In the intestines of the common frog, or 

 toad, may at any time be found a flat, mouthless infusorian known as 

 Opalina ranarum ; it is covered throughout with fine, even cilia. There 

 hatches from the eggs of the coelenterata an animal not resembling 



