iv. 9 NERVOUS SYSTEM OF LAMPREYS 97 



female are enlarged and often red; in addition she has an anal fin, 

 probably used, as in salmon and trout, to make a nest. These sex 

 differences, which develop shortly before spawning, can also be 

 initiated by injection of anterior pituitary extracts (p. 107). 



9. Nervous system of lampreys 



The nervous system of the cyclostomes is very much better 

 developed than that of amphioxus and shows the characteristic plan 

 that is present in all vertebrates. The essence of the vertebrate ner- 

 vous organization may be said to be that it consists of large amounts 

 of tissue and is highly centralized. The brains of vertebrates contain 

 much larger aggregates of nervous tissue than are to be found in any 

 other animals, and this tissue produces by its actions the most charac- 

 teristic features of vertebrate life. Vertebrates are active, exploratory 

 creatures, and their behaviour is much influenced by past experience. 



We shall return later to detailed discussion of the organization of 

 the central nervous system; now we may look briefly at the plan 

 found in the lamprey, as an introduction to that of other vertebrates. 

 As compared with amphioxus there has been a very high degree of 

 cephalization. The front end of the spinal cord is enlarged into a 

 complicated brain, and the nerves connected with a number of the 

 more anterior segments have become modified to form special cranial 

 nerves. 



The spinal nerves, however, still show the plan found in amphioxus 

 in that the dorsal and ventral roots do not join. In amphioxus the 

 ventral roots contain motor-fibres for the myotomes and some 

 proprioceptive fibres, while the dorsal roots contain sensory fibres and 

 motor-fibres for the lateral plate musculature (p. 36). The details of 

 the composition of the nerves of lampreys are still unknown, but there 

 are hints of considerable deviations from this plan. The ventral roots 

 contain many motor fibres passing to the myotomes. The dorsal roots 

 consist largely of sensory fibres with bipolar cell bodies collected into 

 dorsal root ganglia including proprioceptor fibres from the myotomes: 

 it is not known whether the dorsal roots also contain any efferent 

 fibres. In the young larva many of the afferent fibres are the processes 

 of cells lying in the spinal cord (Rohon-Beard cells), which are 

 typical of the early stage of many chordates. There are few types 

 of cells in the cord at this time, allowing for only the simplest 

 reflex arcs. 



The autonomic nervous system shows some generalized and some 

 special features. The gut is mainly innervated by the vagus, which 



