22 Introduction to Animal Morphology. 



lions, epithelium, connective tissue, c., do not. When 

 such elements increase in a heteroplastic body, some 

 sensitive replacement for the primordial protoplasm is 

 necessary : hence cells and fibres are set apart for re- 

 ceiving impressions and conveying irritations. The 

 simplest of such apparatuses are the neuro-muscular 

 cells of Hydra (Fig. 13). Nerve fibres first appear as 

 minute homogeneous bands' (primitive fibrils, as in 

 some Ccelenterates and Annuloida). Such fibres are 

 commonly united in longitudinally striated bundles, 

 with scattered nuclei and an inter-fibrillar granular 

 tissue (naked axis cylinders,* as in many Inverte- 

 brates). These bundles may be invested with a medul- 

 lary sheath of a highly refracting oily substance and 

 protagon. On isolation, these fibres break up and 

 become varicose; after death the sheath coagulates. 

 The fibres of the optic and auditory nerves of Mammals 

 are of this kind. These vary from in^V to in 



diameter. In the ordinary nerves of all Vertebrates (ex- 

 cept Acrania and Marsipobranchii) there is superadded 

 a transparent, structureless, feebly refracting, nucleated, 

 sometimes laminated, connective tissue (nucleated 

 sheath of Schivann\ very thick in the nerves of Electric 

 Fishes. Bundles of medullated fibres enveloped in a 

 nucleated sheath of Schwa tin form the olfactory nerves 

 of Mammalia, or without such a sheath in the olfactory 

 tract of Sharks. Similar fibres separately enveloped 

 in the connective sheath form the abdominal branches 

 of the sympathetic nerves (Rcinak's fibres) in Verte- 

 brates, and are common in Invertebrates. Among 



* (/;vj ;/<//- r describes the a\i> c\limk-i> as made up of transversely placed 

 discs of one material, with a diverse substance intercalated. Schui 

 the axi* cyliiidei- consist of separate fibrils 



