CONTEACTILE VESICLES. 



43 



Fig. 19. 



lies a little to the left of the ossophagus, near the plane of the ciliary 

 disk ; from it a longitudinal 

 vessel runs to the posterior 

 extremity of the animal, 

 and an annular vessel round 

 the ciliary disk, close under 

 its series of cilia. Both these 

 are visible even during the 

 expansion of the contractile 

 vesicle, but swell up sud- 

 denly during its contrac- 

 tion, like the rays of Para- 

 medum above ^described; and 

 then the longitudinal vessel 

 usually|exhibits considerable 

 dilatations, which, when su- 

 perficially examined, might 

 easily be taken for inde- 

 pendent disunited cavities 

 (vacuoles) : the annular 

 vessel exhibits a more uni- 

 form aspect; only two 

 rounded dilatations make 

 their appearance in it. 

 Both vessels gradually de- 

 crease during the reappearance of the contractile vesicle, apparently 

 without any contraction of their own. 



It is therefore probable that in all the Infusoria possessed of a con- 

 tractile space, it is the centre of a vascular system which does not merely 

 consist of chasms formed in the parenchyma by its accidental separa- 

 tion. Another and more difficult question concerning its nature remains 

 to be cleared up, namely whether the vessels and the contractile space 

 possess proper walls in other words, whether the contractile space is 

 or is not a vesicle. The mode of its contraction, which differs from the 

 other contractile phenomena of the parenchyma of the body, appears to 

 speak decidedly in favour of its vesicular character, as do some other 

 facts, such as the phenomenon presented by Spirostomum ambiguum, 

 an animalcule in which the anus is situated at the hinder end of the 

 animal, and close in front of it is the very large contractile space. 

 When fully expanded, this space appears to be surrounded only by a 

 thin membrane ; but nevertheless pellets of excrement, often several 

 at the same time on different sides of the vesicle, form projections 

 which are nearly hemispherical both towards the vesicle and the outer 

 surface of the body, a circumstance which could not happen if it were 

 not a vesicle with proper walls. 



Stentor Millleri (Ehr.), magnified. It is represented 

 in a half-contracted state, and placed in indigo-water, so 

 as to show the ciliary currents. 



