CUTICULAR ELEMENTS. 57 



substance the " endoplasm." The possession of this slightly denser ectoplastic 

 in place of a distinct cuticular layer is evidenced in certain Pantostomata 

 and in many Eustomatous Flagellata and Discostomata, which, while 

 usually exhibiting a more or less characteristic normal outline, can revert 

 at will to a pseud-amceboid and repent state, progressing then through the 

 aid of variously modified pseudopodic prolongations. The possession of 

 a well-differentiated cuticular layer, while regarded usually as the special 

 attribute of the Ciliata, is common also to many Flagellata, being among 

 these latter most conspicuously represented in such genera as Euglena, 

 Anisonema, and Polytoma. 



The development of a simple external or bounding membrane in 

 addition to an immediately subjacent firmer ectoplasm, commonly 

 styled under such circumstances the cortical layer, by no means, how- 

 ever, exhausts the cuticular organization of the Infusoria. As demon- 

 strated by Professor Haeckel, it is possible, among the most highly or- 

 ganized representatives of this class, to recognize no less than four 

 distinct layers or elements exterior to the soft, semifluid, central endo- 

 plasm, the same taking from without inwards the following plan of 

 construction and arrangement. Outermost of all occurs that perfectly 

 hyaline homogeneous layer with which the name of the true cuticle is most 

 appropriately associated. It represents the formed, and consequently life- 

 less, cell-wall of ordinary plant and animal tissues, and is as an independent 

 structure most readily distinguished in such a type as Vorticella, where, 

 in addition to forming the outer envelope of the body proper, it is con- 

 tinued downwards, and constitutes the external, structureless, hyaline and 

 elastic sheath of the characteristic retractile stalk. It is this structureless 

 and transparent external layer, again, which enters into the composition of 

 the more or less indurated dorsal shield or investing cuirass of Ettplotes and 

 Peridinium ; while it is out of this same element, though as a secondary 

 product, that we find derived the hardened cases or loricse of Vaginicola, 

 Tintinnus, and other Heterotricha and Peritricha. Immediately beneath 

 the hyaline external cuticle is encountered, without exception, throughout 

 that large section that takes its name from its characteristic ciliary organs, 

 that comparatively firm, homogeneous, highly elastic and contractile layer, 

 of which the cilia, or their variously modified representatives in the form of 

 setae, styles, or uncini, are the direct products or appendages, and which 

 latter necessarily perforate the external cuticle in order to be brought in 

 contact with the surrounding fluid. With reference to the special func- 

 tion of this element, Haeckel has proposed to confer upon it the title of 

 the ciliary layer. Beneath this last-named layer is found developed in 

 certain of the more highly organized Ciliata, though by no means with 

 a large number, that peculiar hyaline and highly contractile fibrillate 

 structure which fulfils for these unicellular organisms functions analogous 

 with those performed by the muscular tissues of the Metazoa. In 

 recognition of the special properties of this last-named element, it is 



