PARAMEC1UM AURELIA AND AMOEBA PROTEUS. 257 



stitutes the ectosarc (C : e.} : but it forms no definite cuticle, though it is 

 possible that the surface in contact with the water coagulates into a pellicle 

 more resistent than the underlying strata. The interior portion, the endo- 

 sarc (C : /.), is fluent and watery, and by contrast more or less opaque, owing 

 to admixture with various elements partly formed by the chemical activity 

 of the protoplasm itself, partly taken up by it from without l . The sub- 

 stances of the former class, according to Leidy, who has carefully studied 

 this organism, consist of (i) minute granules, pale or dark : (2) spherical 

 corpuscles of largish size, homogeneous character, and either colourless or 

 feebly yellow, liquid or semiliquid : (3) round or oval bodies resembling 

 starch grains both in appearance and chemical reaction : (4) colourless, 

 yellow, or brown oval globules, with dark border, highly refractile and 

 apparently fatty in nature (C :/".): (5) minute crystals and (6) water drops, 

 colourless or feebly yellow. Substances taken up from without are (i) food 

 balls, soft, generally spherical and uniform in size but very variable in colour 

 and composition, according to the nature of the organism (Rotifer, Infuso- 

 rian, &c.) from which they are derived : (2) food materials of a firmer nature 

 such as Diatoms, Desmids, Unicellular Algae, which retain their shape 

 owing to the presence of a skeleton or firm cuticle : (3) various foreign 

 bodies, organic and inorganic, picked up from the surface on which the 

 animal happens to be creeping. The food materials are frequently but not 

 invariably inclosed in a vacuole. They as well as other substances can be 

 taken up at any point in the body, either by the union of two inclosing 

 pseudopodia or by the protoplasm flowing over and around them. Expul- 

 sion of faecal and other materials as a rule takes place at the posterior 

 extremity (C :/.) when the animal is in motion. ' The discharge is rather 

 sudden and is often accompanied with the escape of some viscid fluid ' 

 (Leidy). The rift in the protoplasm closes at once, leaving no trace of its 

 existence. It is a noteworthy fact that, in spite of the soft nature of the 

 inclosing protoplasm, the food-balls, vacuoles contractile and non-contractile, 

 retain their shape and integrity in every movement of the endosarc and 

 animal. 



There is always one nucleus, and sometimes more than one (C : .), and 

 a contractile vacuole. The position of the nucleus varies, but it is 

 generally somewhere about or behind the centre of the body. It is colour- 

 less, homogeneous, finely or coarsely but uniformly granular : in shape a 

 compressed sphere or disc with convex, flat, or concave surfaces and 

 rounded edge. The contractile vacuole (C:c.v.) is a clear globe usually 

 placed behind the nucleus at the posterior extremity. It enlarges slowly 



1 The distinction of the protoplasm into ecto- and endo-sarc is probably, strictly speaking, 

 accidental ; and there is nothing to prove that the ectosarc cannot and does not mix with the endo- 

 sarc. In other words, the distinction is not to be regarded as permanent, in the same sense that the 

 distinction between cortex and medulla in Paramecium is permanent. 



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