AND PHYSIOLOGY OF TRICHODINA. 117 



expanded state, or half expanded (fig. 4), or even shortens its length so much that its 

 body resembles a wheel (fig. 5) rolling on its axis, or turning end for end and performing 

 a series of somersaults with great rapidity. Presently it returns to its more quiet mode 

 of life, sliding spectre-like over the animate surface which forms its principal field of 

 operations. During its act of reptation it revolves very slowly upon its longitudinal axis, 

 as if upon a pivot, and most frequently, if not always, wheels to the right. 



§ 2. Specific Relationship. 



When looking at perfectly fresh and lively specimens of this infusorian, one can hardly 

 believe, at first, that their deep, cyathiform, dice-box-like bodies (figs. 1, 2) are specifically 

 identical with the straight and broad cylindrical forms which are figured by Ehrenberg and 

 Dujardin, or with the turban-shaped bodies which are illustrated in the papers of Stein 

 and Busch ; but when, upon prolonged investigation, we see that the least interference 

 with their freedom of motion causes them to assume a depressed form and a partially 

 retracted margin, we recognize their close resemblance, at least, to those of the above- 

 named authors. Tbe former state represents Nature in reality, the latter exhibits her in 

 a disguised shape. It is therefore with no small degree of reluctance that one concludes 

 to identify the flexible, irregularly funnel-form, conspicuously asymmetrical body of tbe 

 American Trichodina with the seemingly stiff, precisely outlined, cylindrical or conical 

 figures illustrated in European works ; but a careful study of this under various conditions, 

 both in regard to space for movement and the quality of the water, inevitably leads to the 

 conclusion that the European figures represent the creature in an abnormal, or at least a 

 more or less restrained condition ; certainly not in a perfectly healthy state. 



If a Hydra, upon which some of these animals are living, is transferred to a flat watch- 

 glass, and the water is frequently renewed, there is not the least difficulty in studying this 

 infusorian whilst in its fullest degree of expansion, and even with a magnifying power of at 

 least five hundred diameters. In fact it is absolutely necessary that the body should be 

 fully expanded in order to understand the relation and nature of certain parts f its 

 organism ; especially the vestibule and oesophagus, and the contractile vesicle. In a 

 semi-expanded state of the body these parts are confused ; and it becomes impossible to 

 ascertain their character with even the least degree of satisfaction. It is on this account 

 that neither the figures of Stein nor those of Busch give the faintest idea of what the 

 anterior region of Trichodina is like ; and we actually get a better and truer impression 

 of its character from the almost forgotten illustrations of Ehrenberg, than from the more 

 modern, and what ought to be more correct, delineations of this animal. 



§ 3. Form. 



The form of the body is like that of a heavy wine-glass (figs. 1, 2, 8, 14) with a very 

 thick and but slightly expanded base. The plane of the margin of the front — i. c. the 

 peristome (d 1 ) — lies parallel with that of the base, or "adherent organ," and nearly at 

 right angles to the axis of the body. The disc (c, c 1 ), or area encompassed by the vibratory 

 crown (b), is deeply depressed, so that the anterior end of the body, not only externally 

 but internally, is truly cyathiform. In fully expanded individuals the depression of the 

 disc extends nearly to half the depth (at c) of the body, and occupies at least nine tenths 

 of the diameter of this region. At times the animal suddenly recurves the edge of the 



MEMOIRS BOST. SOC. NAT. HIST. VoL. I. 30 



