394 GENERAL HISTORY OF THE INFUSORIA. 



tissue abounds, the vacuolar thickenings have been conceived to represent 

 ganglions or, otherwise, glands. 



The cuticle, or external limiting membrane of the integument, is hardened 

 by the deposition in it of the peculiar chemical principle chitine, the same 

 which imparts firmness to' the covering of Entomostraca, Insects, and other 

 Articulata ; or if not actually chitine, it is a substance closely allied thereto. 

 This is Leydig's opinion, and it seems sufficiently confirmed by the reaction 

 of chemical agents. Thus, he shows that caustic alkali (potash) does not dis- 

 solve the cuticle when it possesses, as it usually does, moderate firmness, in 

 other words, when an infusion of chitine exists in its substance. But when 

 the animal lives within an external case, and does not need the protection of 

 an immediately investing skeleton, the chitine is absent, and the integument 

 dissolves in the alkali. The analogue of this may be found among the 

 Articulata. 



The prevalent opinion has been, that the dense cuticle or external skeleton 

 of Rotifera differed from that of the Crustacea and other Articulata in not 

 being of a chitinous nature ; and this hj^othesis was used in arguments 

 relative to the affinities of the Rotatoria. Thus Kaufmann advances it as a 

 decided distinction between this class and the TardigTada ; but, as Leydig 

 remarks, the skin of the latter animals is even more affected by potash than 

 that of the Rotifera {see section on the Affinities of Rotatoria). 



The cuticle, as just intimated, differs much in firmness and thickness in 

 different species. It is softest in those which live in an external case — e. g. 

 Stephanoceros, Melicerta, Tuhicolaria, — and in such as are invested by a 

 gelatinous sheath — e. g. Notomyyiata centrura. In Diglena, Notommata aurita 

 (XXXVI. 3, 4), Asplanchna, and others, it is firmer, but still flexible ; 

 whilst in such genera as Bracliionus (XXXIX. 16, 17, 21), Noteus, Salpina, 

 and Euchlanis (XXXIX. 4) it attains a rigid, horny consistence, resembHng 

 that of the shells of Entomostraca. Even where the skin is of considerable 

 firmness, it is yet capable of distension, as Perty observed in the Scaridium 

 longicaudum when its stomach was stretched with food. The form of the 

 body is much modified by the degree of firmness of the integument. When 

 this is soft and yielding, or flexible, the figure is rounded, and more or less 

 elongated, and may taper towards one or both extremities ; but when the 

 cuticle is much hardened, the rounded configuration is often lost, and various 

 irregularities in form result. For example, in Metopidia and in Euchlanis 

 dilatata (XXXVIII. 5) the body is ovate and compressed, or depressed ; in 

 Emhlanis triquetra it is triangular (XXXIV. 443) ; in E. hipposideros and in 

 Lepadella (XXXIV. 430-432) the dorsum is convex, the abdominal surface 

 flat ; in Noteus quadricornis (XXXVIII. 25-27), suborbicular and com- 

 pressed ; in Mastigocerca carinata and Ratulus carinatus (XXXIV. 438-440) 

 it is prismatic, with one angular ridge or crest ; in Colurus, compressed 

 laterally. 



There is, besides, a direct relation between the segmentation of the integu- 

 ment, the perfection of its articulate condition, and the degree of firmness of 

 the integument. The soft- skinned Rotatoria only throw their bodies into 

 folds during contraction, whilst those with firmer cuticle, such as Philodincea, 

 develope the sliding joints, and, lastly, those {e. g. Lepadella and Euchlanis) 

 which have a dense homy covering present two or three decided segments, 

 recalling in form and disposition the di^dsions of the external skeleton of the 

 monocular Entomostraca, or even of the higher Crustacea. 



Where the cuticle is condensed into a rigid, horny lamina, defending the 

 animal like the shell of a Crustacean or the carapace of a tortoise, it may 

 well be termed a testa, testula, or lorica. This last name was very loosely 



