86 COMPAEATIVE ANATOMY. 



partly constant. Wlien they are constant their function is in- 

 creased, and they often expand and contract regularly and rhyth- 

 mically, like the cardiac systole and diastole. Contractile vesicles 

 of this kind are often seen in the Amoeba (Difflugia and Arcella), 

 and are very common among the Infusoria. They are also known 

 as vacuoles. The fluid which collects in the vesicles is drawn from 

 the parenchyma of the body and is returned to it, or passed out to 

 the exterior on the contraction of the vesicle. Fine communications 

 with the exterior have been made out, so that the latter course is the 

 probable one ; but we need on this account conclude that water does 

 not enter by the same passage. 



In the Infusoria the vesicles lie in the cortical layer (Fig. 28, dd), 

 generally just under the delicate cuticle, and at definite points. If 

 only one vesicle is present, it lies either anteriorly or posteriorly : 

 if two, there is one near each end of the body. Trachelius ovum is 

 remarkable for a large number of small vesicles. 'No special mem- 

 branes can be made out on the wall of the vesicle nor in the canals 

 which pass off from it. Like the vesicle the canals can only be 

 made out while they are filling. The vesicle and canals contract 

 alternately. In Paramtecium the canals enlarge at the commence- 

 ment of the systole, and approach one another as the vesicle diminishes 

 in size, so that they form a stellate figure at the moment when its 

 systole is most complete and the vesicle has disappeared. While the 

 vesicle is filling the canals look like small diverticula on it, and are 

 not again fully distended until the diastole is complete. The number 

 of canals, which is limited in P. aurelia to eight or ten, is increased 

 to thirty in Bursaria flava, and is much higher in Cyrtostomum 

 leucas. In these forms the canals have a wave-like course, and ramify 

 at their extremities. Canalicular tracts are formed by the fusion of 

 several spaces fiUed with water into longer tracts, as in Stylonychia 

 (St. mytilus), and they empty themselves into the contractile vesicle, 

 by definite passages. The long canals of Spirostomum ambiguum, 

 which also are visible for a time only, but which are longer 

 than these, are like them, so that we can make out a continuous 

 series from the first appearance of an apparently indifferent cavity to 

 a definitely arranged system of tubes. 



Another arrangement can be put beside this formation of in- 

 different vacuoles. When such spaces in the protoplasm increase 

 in number they run together, and so give to the protoplasm the 

 appearance of a network which traverses the interior of the body, 

 which is filled with fluid (Trachelius ovum). These hollow spaces 

 have then become perfectly different organs from the pulsating 

 vacuoles, and they may both exist at the same time. 



§69. 



In correspondence with their low grade of organisation the 

 Protozoa have no sexual organs, and give but the faintest indica- 



