62 



ZOOLOGY 



SECT. 



After leading an active existence for a longer or shorter time, the protoplasm 

 aggregates into a solid lump, surrounds itself with a cyst, and undergoes multiple- 

 fission, dividing into an immense number of minute spores. The cyst (Fig. 45, 

 A, spy. 1, spg. 2) is therefore not a mere resting capsule, like that of Amoeba, 

 but a sporangium or spore-case. Its wall consists of two layers, an inner of a 

 dark purple colour and membranous texture, formed of cellulose, and an outer of 

 a pure white hue, formed of calcium carbonate. Thus the whole sporangium, 



FIG. 45. Didymium difforme. A, two sporangia (spg. 1 and 2) on a fragment of leaf(/.); 



B, section of sporangium, with ruptured outer layer (a.); and threads of capillitium (cp.); 



C, aflagellula with contractile vacuole (c. vac.) and nucleus (JIM.); D, the same after loss 

 of flagellum ; b, an ingested Bacillus ; E, an amcebula ; F, conjugation of amoabulse to form 

 a small plasmodium ; G, a larger plasmodium accompanied by numerous amosbulte ; sp. 

 ingested spores. (After Lister.) 



which may attain a diameter of 3 or 4 mm., resembles a minute egg. From the 

 inner surface of the wall of the sporangium spring a number of branched 

 filaments of cellulose, which extend into the cavity among the spores and together 

 constitute the capillitium (B, cp. ). 



The spores consist of nucleated masses of protoplasm surrounded by a thick 

 cellulose wall of a dark reddish-brown colour. After a period of rest the proto- 

 plasm emerges in the form of an amoeboid mass which soon becomes a flagellula 

 (C), provided with a single flagellum, a nucleus (mi.), and a contractile vacuole 



