28 



LOWER IN VER TEBRA TES. 



'. — Splrostomum ambujuum greatly eularged. 



organ is enclosed in a membrane. The func- 

 tion of the nucleus will be referred to fur- 

 ther on. 



The contractile vacuole is 2:)resent in the 

 majority of species. There may be only 

 one (the usual condition), or several, and 

 in rare instances many; in Amphileptus 

 wiser there are from ten to fifty, arranged 

 in two longitudinal series. It is an interests 

 ing fact that the pulsations of these vacuoles 

 occur, first on one side, and then on the 

 other, progressing from the anterior to the 

 posterior end of this elongate form. The 

 vacuole may be simple or quite complex. 

 When simple it is more or less spheroidal ; 

 in an active animalcule under the micro- 

 scope it may be seen to steadily fill with a 

 clear, watery fluid; when the full size is 

 reached it suddenly collapses, after which, 

 often, no trace of it appears until it begins 

 again to fill ; the pulse is rhythmical when 

 the animal is expanded seeking food. 

 Among the complex types may be men- 

 tioned those spherical forms with two or 

 more radiating sinuses or diverticula. In 

 Paramecium aurelia (Fig. 37), there are 

 usually from five to eight of these blind 

 canals; in Stentor polymorphus (Fig. 42, 

 ci'.), a sums extends from the bulb situ- 

 ated near the anterior border down to the 

 foot, while another branch extends from 

 the bidb about the jieristome ; in Sjnros- 

 tomum ambiguum (Fig. 22) it is somewhat 

 similar to that in Stentor, taking the form 

 of a lateral canal with a very large bulb 

 at the posterior extremity; it is often en- 

 larged also at the opposite end. Con- 

 cerning its use, whether it is a true organ 

 with bounding walls, or whether it connects 

 with the external water, there has been 

 much lively dispute; indeed, students are 

 still divided in their opinions of these ques- 

 tions. Ehrenberg regarded it a spermatic 

 gland, Dujardin attributed to it a respira- 

 tory function, Claparede and Lachmann a cir- 

 culatory function. Stein excretory. Huxley 

 remarks that its function is entirely unknown, 

 though it is an obvious conjecture that it 



