26 LECTURE II. 



mytilus, and Euplotes patella, the pulsating sac is single and sub- 

 circular, and situated at one side of the body. In Actinophrys, 

 Sursaria and Trichod'ma there are two pulsating sacs : in Loxodes 

 Bursaria one of them is situated in the anterior third, tlie other in 

 the middle third part of the body {fig' 18. v, v.). In Arcella vulgaris 

 from three to four pulsating sacs have been observed. In Nassula 

 elegans four round hearts follow one another along the dorsal region 

 of the body ; in Trachelius Meleagris there is a row of from eight 

 to twelve such hearts. The contractile sac presents the form of a 

 long pulsating vessel in Spirostomum ambiguum and in Opalina 

 Planariarum. In Paramcecium Aurelia canals extend from the 

 circular sac in the form of rays. The Cryptomonas ovata and Opalina 

 Planariarum are the only species of the Astomatous Infusoria in 

 which the contractile sac has been observed.* 



By the analogy of the gills of the acephalous Mollusks we may 

 regard the mechanism for renewing the surrounding oxygenised 

 medium upon the respiratory surface, to be the superficial vibratile 

 cilia, the action of which upon the water is necessarily attended in 

 the free Infusoria with a reaction which rolls the little animalcule 

 through its native element and produces the semblance of a definite 

 voluntary movement. 



In the saline springs at Konigsborner and Rodenberg, Wohlerf ob- 

 served that the green mantle, formed of innumerable individuals of 

 Frustalia salina, Chlamydomonas pulvisculus and Euglena viridis, 

 was raised from the surface, here and there^ by bubbles of gas. He 

 collected the gas, and found it to be almost pure oxygen. This in- 

 teresting discovery clearly indicated a respiratory process like that of 

 plants. The minute green corpuscles, partly fixed in the inner layer 

 of the integument, partly circulating in the contained fluid of the 

 Polygastria, and regarded by Ehrenberg as ova, correspond in their 

 nature with the chlorophyll corpuscles of AlgcE, and most probably 

 perform an analogous function, fixing the carbon of the atmosphere 

 in the hydro-carbonates, and evolving oxygen. 



Perhaps the most marvellous part of the organisation and economy 

 of the Polygastric Infusoria is that which relates to the function of 

 generation : the only one which does not necessarily require a special 

 organ for its performance — a proposition which will be quite intelli- 

 gible when the essential nature of the generative process is understood. 



The part which Ehrenberg describes as the " testis " is the usually 

 large sub-spherical corpuscle (Jig. 17. n.\ situated at or near the 

 middle of the body in the Polygastria. It presents an extremely 



* XXIV. p. 20. t VIII- 1843, p. 206. 



