42 



INFUSORIA. 



(87.) The contractile space (seminal vesicle according to Ehrenberg) 

 is regarded by many recent authors, in accordance with Dujardin's 

 example, as a cavity destitute of proper walls (vacuole), which is some- 

 times supposed to form the analogue of a heart, and sometimes that of 

 an excretory or respiratory water- vascular system. In order to be able 

 to judge of these views, it will be necessary to examine rather closely 

 into the behaviour of this " contractile space ;" and for this purpose those 

 Infusoria in which processes or branches derived from it can be detected 

 appear to be particularly worthy of attention. 



Fig. 18. 



Paramecium aurelia (Ehr.), magnified 300 diameters. 1. The animalcule at rest, under slight 

 compression: o', the position of the mouth; s, s, contractile vesicles surrounded with radiating 

 processes ; t, nucleus. 2. Another specimen, placed in water coloured with indigo, showing the 

 ciliary currents, the oral aperture, o', and the coloured pellets in the interior of the body. 3. The 

 same in process of longitudinal division, each segment containing a distinct nucleus. 4. A spe- 

 cimen swimming freely about after being fed with carmine ; the mouth, nucleus, and star-like con- 

 tractile vesicles are seen more in profile. 



(88.) Radiating branches of the contractile spaces were first discovered 

 by Ehrenberg in Paramecium and some other Infusoria. When the con- 

 tractile space is fully expanded, the rays can only be observed as fine 

 lines, or, when the light is not good, are entirely imperceptible. On 

 the sudden contraction of the space, however, they instantly swell into 

 pyriform cavities placed close to the position of the contractile organ 

 which has disappeared. During the slow reappearance of the contractile 

 space, the rays gradually decrease, and they in turn have almost disap- 

 peared or become reduced to fine lines when the vesicle has attained its 

 full extension. These rays, as well as the contractile spaces, lie, as in 

 all Infusoria, close under the skin (" cuticula" of Cohn), in the paren- 

 chyma of the body (" cortical layer " of Cohn). 



(89.) The processes of the contractile space are seen with remarkable 

 clearness in the large Stentor polymorphus, in which a very considerable 

 portion of a vascular system may be recognized. The contractile space 



