THE INFUSORIA 



365 



medullary substance. To these the terms exoplasm and endoplasm 

 respectively are usually applied by authors, but, as pointed out 

 by Lankester, it is important that the permanent arrangement of 

 the living substance in the INFUSORIA should not be confounded 

 with the transitory arrangement of the particles forming the 

 exoplasm and endoplasm of the GYMNOMYXA, and it is therefore 

 advisable to retain in this connection the terms "cortex" and 

 " medulla." 



CORTEX (Ectosarc, Ectoplasm). The cortex is, in its simplest 

 form, a clear, firm outer layer of protoplasm, bearing the cilia or 

 suckers, and showing no evidence of further differentiation even 

 with the highest powers of the microscope. In some of the larger 

 Heterokaryota, however, three distinct layers may be recognised 

 an outer very thin skin, called the pellicle ; a lower layer, usually 

 characterised by a series of vertical and parallel lines, called the 

 alveolar layer ; and an inner layer of clear transparent protoplasm 

 continuous with the medulla, and distinguished from it only by 

 the scarcity of granules and the absence of food vacuoles. 



These three layers are in all cases organically continuous, and 

 are rightly regarded as being built up of living substance. 



The pellicle is rarely more than 1 //, in thickness, but most 

 frequently it is so extremely thin as 

 to be only just distinguishable in 

 section. It is apparently very tough, 

 and where it is grooved or ringed 

 it determines the outline of the 

 body. In some forms it gives rise 

 to hard hook-like processes, as in the 

 parasite Trichodina, or to a mailed 

 armature, as in Coleps (Fig. 5). 



The alveolar sheath is usually 

 much more apparent than the pell- 

 icle, and is very frequently marked 

 by longitudinal lines which are 

 regarded as indications of specialised 

 strips of contractile protoplasm. It 

 is only in a few forms that these 

 strips are sufficiently differentiated 

 to be distinguished as specialised 

 muscular fibrils. In such cases, how- 



Fia. 5. 



cdeps 



Bhr> one of the mailed 

 A, one of the 15 rows of 



ever, they are enclosed in a canal Hoiotricha. 



i ,i ,i -i F plates. (From Butschli, after Maupas.) 



sunk beneath the general surface x 900. Freshwater. 



of the alveolar layer, and are called 



myoneme threads (Fig. 6). Of great interest are the myoneme threads 



occurring in particular parts of the body of some genera such as 



the sphincter of the peristome in Epistylis, the " myophan " thread 



