NERVOUS SYSTEM 149 



anal vesicles, and the crocodiles rather large glands, called 

 musk glands, opening into the cloaca near the anus. 



In the opinion of Diisselhorst the cloacal glands of the male 

 Urodeles may be compared to the prostate, but up to the 

 present nothing has been recorded as to the abdominal and 

 pelvic glands and their functions. 



VIII. Nervous System. 



The faculty of receiving impressions from the outer world 

 is common to all organisms up to man, but the possession of 

 special organs for perceiving and conveying sensations (nerve 

 fibres and nerve cells) does not extend to the lowest grades of 

 the organic scale, for, though undoubtedly capable of sensation, 

 the plants, the Protozoa and the lowest of the Ccelenterata 

 (fixed Hydroidae, Lucernaria, Anthozoa) are constructed of 

 tissue so differentiated as to exclude the possibility of the 

 ultimate development of a nervous system. Such a structure 

 is first found in the higher Ccelenterata, the Ctenophora and 

 Medusae. The nervous system of the Ccelenterata, so far as 

 has been ascertained, consists of ganglia, or small aggregations 

 of nerve tissue, in the fundus of the digestive cavity, and nerve 

 fibres proceeding from the ganglia. In the Medusae the system 

 takes the form of a ring following the margin of the disc, and 

 consisting of ganglia placed at regular distances, and nerve 

 fibres connected with these ganglia. Such simple structures 

 represent the fundamental type on which the nervous system 

 of all organisms, from the Invertebrates up to man himself, is 

 constructed. The nerve ganglia act as central organs receiving 

 sensations by means of certain nerve filaments, and promoting 

 action by means of others. The simplest example of this 

 structure may be seen in those worms in which the body is not 

 segmented (metameres) ; where, however, the body is articu- 

 lated an almost uniform repetition of the central organs 

 (ganglia) takes place. Moreover, the chief ganglia are, in all 

 the worms, situated in the anterior part of the body, and herein 

 we recognise the prototype of the central nerve organs of the 

 animals of all the higher classes and orders. 



In the Echinodermata nerve trunks tapering towards the 



