RESPIRATION — DIGESTION I 5 



cuticle, a vacuole forms at the point, grows and distends the over- 

 lying cytoplasm, which finally ruptures : the walls of the vacuole 

 disintegrate ; and this goes on as above described. Ciliate Infusoria 

 are especially liable to this disintegration process, often termed 

 " diliiuenee," which, repeatedly described by early observers, has 

 recently been studied in detail by Verworn. Here we have death 

 by " solution," while in the " fixing " of protoplasm for microscopic 

 processes we strive to ensure death by " desolution," so as to retain 

 as much of the late living matter as possiljle. It would seem 

 not improbable that the unusual contact with water determines 

 the formation of a zymase that acts on the living substance itself. 



We have suggested ^ that one function of the contractile 

 vacuole, in naked fresh-water Protists, is to afford a regular means 

 of discharge of the water constantly taken up by the crystalloids 

 in the protoplasm, and so to check the tendency to form irregular 

 disruptive vacuoles and death by diffluence. This is supported 

 by the fact that in the holophytic fresh-water Protista, as well 

 as the Algae and Fungi, a contractile vacuole is present in the 

 young naked stage (zoospore), but disappears as soon as an 

 elastic cell-wall is formed to counterbalance by its tension tlie 

 internal osmotic pressure. 



Digestion is always essentially a catabolic process, both as 

 regards the substance digested and the formation of the digesting 

 substance by the protoplasm. The digesting substance is termed 

 a " zymase " or " chemical ferment," and is conjectured to be pro- 

 duced by the partial breakdown of the protoplasm. In presence 

 of suitable zymases, many substances are resoh'ed into two or 

 more new substances, often taking up the elements of w^ater at 

 the same time, and are said to be " dissociated " or " hydrolysed " 

 as the case may be. Thus proteid substances are converted into 

 the very soluble substances, " proteoses " and " peptones," often 

 with the concurrent or ultimate formation of such relatively 

 simple bodies as leucin, tyrosin, and other amines, etc. Starch 

 and glycogen are converted into dextrins and sugars ; fats are 

 converted into fatty acids and glycerin. It is these products of 

 digestion, and not the actual food-materials (save certain very 

 simple sugars), that are really taken up liy the protoplasm, 



1 In Rej}. Brit. Ass. 1888, p. 714 ; Ann. Mag. Nat. Hist. (6), iil. 1889, p. 64, 

 This view has been fully worked out, mainly on Ciliates, by Degen in Bot. Zeit. 

 kiii. Abt. 1. 1905. 



