LOWER INVERTEBRATES. 



Branch I. — PROTOZOA. 



In the pages of the Introduction we have a definition of a cell, with a brief account 

 of the part it plays in the structure of animals, and now in the Protozoa we are to 

 study the manifestations of cell life in their simplest forms ; for these animals during 

 their whole existence consist each of but a single cell ; yet, simple as this structure 

 would seem to be, we find manifestations of almost all vital phenomena exhibited by 

 these forms. Every member of the branch has the power of motion, of assimilating 

 food, and of reproducing its kind, all of these functions being performed by the single 

 ceU. 



In the Cuvierian system of classification no place was accorded to this group, for 

 they were either regarded as embryonic forms, or, as in the case of the Foraminifera, 

 they were transferred bodily to some of the four great divisions into which the animal 

 kingdom was divided. Though the Protozoa have been studied for over two hundred 

 years, it was not until 1845 that they were first considered as unicellular forms, and for 

 a long time after that date the most prominent naturalists refused to accept the con- 

 clusions of the illustrious von Siebold. Ehrenberg, who studied these forms very 

 thoroughly, and in 1838 published a large and extensively illustrated work upon them, 

 describes with great detail nervous, digestive, motory, reproductive and sensory sys- 

 tems in these really simple organisms, all of which have since been shown to have no 

 actual existence. These mistakes, great as they now appear, arose very naturally, for 

 at the time at which Ehrenberg wrote, Schwann had not made known his studies upon 

 cells ; and highly preposterous at that day would seem the idea that an animal could 

 exist without definite organs to perform the functions of animal life. Were space at 

 our disposal, it would prove an interesting chapter to review the history of the disputes 

 regarding the character of these forms, the rash and dogmatic assertions of prominent 

 naturalists who believed that there could be only the four great divisions of the animal 

 kingdom which the great Cuvier had proposed, and, on the other hand, the patient 

 observations and the guarded statements of their opponents. Time, however, served to 

 clear up the doubts surrounding these minute forms, and to-day not a naturalist lives 

 who does not in some way accept the group. 



The Protozoa are mostly microscopic animals consisting of but a single cell, or, in 

 a few cases, apparently of an association of cells, without, however, any differentiation 

 into tissues. These few apparent exceptions will be considered more at length further 

 on. In some of the Protozoa the cell is provided with a nucleus and various other 

 differentiations of the protoplasm ; in others no such structures have as yet been dis- 

 covered, the animal, so far as our knowledge enables us to say, being but a cytode, a 



VOL. I. — 1 



