GROMIA 



34^ 



can be no doubt whatever. Not only do the body, stem, and cilia of the vorticella move in a co-ordinated manner 

 but the several parts of the body move ; each part having the power of contracting centripetally and dilating centri- 

 fugally. This power of changing shape in every part is vouchsafed to all, or nearly all, the rudimentary forms. The 

 movements in the vorticella and other rudimentary animals are spontaneous and independent, and are not due to 

 reflex action, irritabiUty, or extraneous stimulation of any kind. Neither are they, in any sense, caused by elasticity 



The pulsating vacuole displays centripetal and centrifugal rhythmic movements similar to those seen in the 

 amoeba, paramecium, &c. 



There is no more striking phenomenon in living organisms than is supplied by the throbbing or pulsating move- 

 ments in question. They appear equally in the simplest and most complex animals, and are, in a sense, coeval with 

 life itself. 



The vorticella reproduces itself by longitudinal division. A constriction or dividing hne appears at the distal 



Fig. 75. — A group of Vorticellse magnified 220 diameters (after 

 Marshall and Hurst). 



A, B, 0. Show the animal in various positions. 



D. A much smaller specimen drawn to the same scale. 



B. A specimen made to contract by the action of alcohol. 



F. A specimen detached from its stalk and swimming away 

 freely, disc forward. 



G, H, I. Illustrate three stages of fission. 



J, K, L. Show the separated individual swimming by means 

 of the aboral circle of cilia — K being slightly contracted, and D 

 strongly contracted, a, Food-vacuole discharging contents at anus ; 

 h, aboral cilia ; c, cilia of the disc ; (/, contractile fibres of the 

 stalk ; d, disc ; d', disc retracted ; fv, food vacuoli ; ph, pharynx ; 

 pv, pulsating vacuole ; ^j, peristoma ; vt, myophan striation ; 

 n, nucleus ; v, vestibule. 



The Vorticellse are remarkable for their beautiful spiral stems 

 and cilia, and the rapidity and grace of their movements. They 

 retract and shorten their stems spirally and suddenly, and elongate 

 them in similar fashion, slowly, warily, and as if feeling their way. 

 The cilia of the cup-shaped discs are continually in motion, and 

 convey the idea of rotation. The movements of the stems and 

 cilia are voluntary, and under perfect control, as any one may readily 

 satisfy himself by careful microscopic examination. The movements 

 are necessary to secure food. They are essentially centrifugal and 

 centripetal, give-and-take, rhythmic movements. 



Fig. 75. 



end of the body, and extends downwards in the direction of the stem. The body is thus separated into two, both 

 halves remaining on the stem. The one half resembles the parent and remains on the stem ; the other, while still 

 adhering to the stem, develops, near its base, an aboral circle of cilia, by the aid of which, when it has detached 

 itself, it swims away. When a separation is effected the stemless half develops a stalk and fixes itself, the aboral 

 circle of ciha disappearing. The interesting process of fission is completed in from one to two hours. A con- 

 verse process is occasionally witnessed, namely, the conjugation or union of two vorticellse. In such cases a small 

 vorticella becomes fused with a larger one, of which it remains a permanent part. Not unfrequently after conjuga- 

 tion, and probably as a result of it, a process of encystment takes place. In this case the disc of the vorticella is 

 retracted, and the cell body becomes rounded and separated from its stalk. Coincident with these changes a cyst 

 is secreted and important changes occur in the nucleus. The nucleus enlarges, elongates, and becomes monihform, 

 and ultimately breaks up into a number of spores, which are ovoid bodies provided with a circlet of ciha. When 

 the cyst bursts the spores escape and swim freely about. They multiply by fission. Ultimately they fix themselves 

 by their ciliary processes, and develop a disc at the other end of the body. The circlet of cilia then disappears, and 

 a stem resembhng that of the parent is formed. This done, each spore becomes a perfect individual. 



The movements, the method of feeding, and the modes of reproduction in vorticellae are all very exact and per- 

 fect in their way. They are plainly the outcome of pre-arrangement and design, and this fact, in conjunction with 



