156 COMPARATIVE PHYSIOLOGY 



latter are cut a purely local response is evoked. The generalised 

 response which occurs when the stellar nerves are intact is 

 obtained equally well after section of the pallial commissures 

 so that the stellate ganglion is completely isolated from the 

 rest of the C.N.S. It follows that the stellate ganglion is 

 a centre of reflex activity. Generally, however, reflexes make 

 use of paths in which at least one intermediate (internuncial 

 or proprio-spinal in the vertebrate) neurone between the 

 afferent and efferent elements is involved. This condition is 

 also seen in Fig. 38. 



The more specialised forms of perception concerned with 

 phototropic, geotropic, and chemotropic reflexes discussed 

 below involve, in practically all cases, separate terminal 

 organs, receptors, in connexion with the peripheral ends 

 of the afferent neurones. A great deal has been written 

 about the anatomy of chemoreceptors, and photoreceptors. 

 So little is known of the mechanism of these structures 

 in vertebrates that the scope of the present work does 

 not permit of more than a reference to Winterstein's " Ver- 

 gleichende Physiologic " for further information. A brief 

 reference is, however, due to the receptors for space orien- 

 tation, which are essentially alike throughout multi-cellular 

 animals (and one might almost add plants where, however, 

 they are unicellular in structure). The statocyst is essen- 

 tially in all cases a sac lined with cells in connexion with 

 nervous elements, enclosing a solid body with sufficient 

 space to move freely under the influence of gravity, which 

 brings it to rest in such a position as to stimulate one or 

 another group of nerve-endings according to the position of 

 the body in space. Kreidl's (1893) ingenious experiments 

 in replacing the statoliths of Crustacea by iron or nickel filings 

 during ecdysis, showed that when the normal mechanical 

 effects of gravity are replaced by those of the magnet the indi- 

 vidual behaves with reference to the field of magnetic attraction 

 precisely as it should on the hypothesis that equilibration 

 depends on the group of receptors on which the statoHth 

 impinges. 



The central nervous system usually consists of more or 



