and their Affinity with the Infusoria flagellata. 1 5 



Several of these genera are new to science, and, moreover, of the 

 most remarkable forms. I regret that words alone cannot, at 

 this time, render their peculiarities as evident as I hope the 

 illustrations will in my forthcoming paper in the ' Memoirs of 

 the Boston Society of Natural History/ 



I must ask the reader, in the first place, to go back with me 

 almost to the Ultima Thule of animal simplicity, and revise the 

 organization of the hitherto too lowly estimated Monas, in order 

 to lay the foundation for the group which embraces in its limits 

 80 gigantic a family as the Spongise ciliatse. I do not think 

 any one will be prepared to fully appreciate such a remarkable 

 definiteness and system in the arrangement of the organization 

 of Monas as I have discovered among the various forms which 

 constitute that genus. 



Hitherto a Monas has been looked upon as a mere shapeless 

 molecule, with a vibrating cilium of some sort or other, attached 

 to its surface at an indefinite point. As I understand the rela- 

 tion of parts now, the motory cilium or flagellum is perhaps the 

 most remarkable feature of the whole animal, not only in a 

 physiological aspect, but also in its topographical relationship. 

 Let me illustrate this by a description of the body and append- 

 ages of Monas termo, Ehr. 



The body of that species has the form of a wide, compressed 

 heart, with two distinct summits. The broad flattened sides lie 

 opposite to each other, and parallel with the plane which passes 

 through the two summits, and which forms the prolongation of 

 the greater transverse diameter of the body. Between these sum- 

 mits is an aperture which constitutes the mouth. One of the 

 summits is prolonged into a broad, conical, beak-like body, and 

 assists the mouth in the prehension of food. It is therefore a 

 true lip. The flagellum^ however, is the real prehensory organ, 

 although it, at the same time, performs the office of a propelling 

 agent when the body is detached from its pedicel and moves 

 about in a free state. This organ has the form of a scarcely 

 tapering bristle, which is attached close to the edge of the mouth, 

 on that side of it which is opposite to the lip, and rises with a 

 decided well-defined curve whose plane is coincident with the 

 plane which runs through the two summits, and forms, as I have 

 just mentioned, the plane of the greater transverse diameter of 

 the body. This remarkable feature is scarcely to be recognized 

 during the free state of the animal ; but when the latter is moored 

 by its posterior end to its pedicel, the phenomenon in question 

 is very marked and conspicuous. For most of the time thefla- 

 yellum sustains itself in this rigid arcuate position, and is always 

 curved away from the lip ; but its terminal end keeps up an 



