76 THE INVOL UNTAR Y NER VO US S YSTEM 



peculiarities which I think worth pointing out in the hope that 

 subsequent research will decide the value of these signposts. 

 The most thorough and detailed account of the structure of a 

 ganglion In Astacus, is that given by Hardy. He points out 

 that three pairs of nerves arise from each of the first five ab- 

 dominal ganglia: 



1. An anterior pair which arise directly from the ganglion 

 and contain a large number of the fine or afferent fibres and 

 comparatively few of the larger or efferent fibres. These supply 

 the appendages with motor and sensory fibres and also the skin of 

 the sternum and pleura. 



2. The posterior ventral nerves containing relatively more 

 large fibres. These supply the dorsally placed extensor muscles 

 of the segment and the dorsal skin in the next following segment. 



3. Posterior dorsal nerves which are purely motor and in- 

 nervate the flexor muscles of the segment. 



The motor fibres in these nerves are very large and few, and 

 each arises from a large motor cell in the ganglion. Thus in the 

 second abdominal ganglion the posterior dorsal nerve consists of 

 only ten large fibres which innervate all the flexor muscles of 

 that segment. Each of these large fibres divides repeatedly on 

 its way to the muscles, ultimately splitting into many small fibres 

 which reach their destination in the muscles themselves. In fact 

 each motor nerve cell and fibre resembles the large motor cell 

 and fibre of the torpedo electric organ, which itself is a modi- 

 fication of a muscular mass. This appears to be the rule in the 

 crustaceans and annelids ; a single large cell in the central nervous 

 system innervates a large number of muscles by the brush-like 

 splitting up of its large axis cylinder process. The process of 

 evolution has brought about the large brain development of the 

 vertebrate, and the greater niceties of voluntary movement have 

 necessitated the more separate innervation of muscle fibres by 

 the smaller cells of the anterior horn. Again in this instance, 

 embryology, as is always the case, recapitulates the phylogeny 

 and shows how with the proliferation of the muscles of the 

 myotomes, there is a splitting up of their motor nerves to form a 

 brush-like termination supplying the rapidly proliferating muscles. 



The most beautiful and clear evidence of this process is given 

 by Miss Alcock's discovery. The adult lamprey Petromyzon 

 possesses a large and elaborate sucking apparatus which consists 



