NERVOUS SYSTEM AND BEHAVIOUR 445 



activity of an organ. The demonstration of so many adrenaline-sensitive 

 tissues in invertebrates poses the problem whether adrenaline is a normal 

 mediator in these animals, or whether sensitivity is a fortuitous accompani- 

 ment of other biochemical activities (6a, 3>la, 78). 



Neurosecretion 



The nervous system usually exerts regulatory control over body activities 

 by means of efferent fibres, which carry signals to the end-organs concerned. 

 But many instances are now accumulating to show that nervous regulation 

 may be more indirect, involving release of hormones — neurosecretion — by 

 specially modified neural structures. The posterior pituitary gland is an 

 example of an endocrine body of nervous origin, and many analogous 

 instances occur among invertebrates. Cephalopods show an interesting 

 transition, in which the giant neurones of the stellate ganglion of decapods 

 become transformed into the neurosecretory cells of the epistellar body of 

 octopods. Special secretory cells have been found in the nervous systems of 

 many groups, including nemertines, annelids, molluscs, crustaceans, fishes 

 and other vertebrates. 



Among polychaetes aggregations of neurosecretory cells have been 

 recognized in the supra-oesophageal ganglia and nerve cord of Nereis and 

 Aphrodite. Modified nerve cells containing secretory granules occur in 

 cerebral and visceral ganglia of opisthobranchs (Aplysia and Pleuro- 

 branchaea). 



Neurosecretory systems are better known in arthropods. In the Mala- 

 costraca there are well-defined groups of neurosecretory cells in the thoracic 

 ganglia, brain and eyestalks (Fig. 10.20). These cells are modified neurones, 

 and their axons run to special storage organs. One of these is a sinus gland 

 associated with the optic lobe, another lies on the course of the post- 

 commissure nerve. The storage organs are made up of the swollen terminals 

 of the neurosecretory axons, which contain abundant secretory material 

 (prawns, crabs, etc.). Large aggregations of neurosecretory cells are also 

 found in the c.n.s. of Limulus (4, 11, 12, 14a, 29, 42, 68, 68a, 97). 



Other peripheral elements of a neurosecretory nature have been des- 

 cribed in Crustacea (stomatopods, decapods). Such are the neuropile net- 

 works known as the pericardial organs that lie in the wall of the pericardial 

 sinus. The pericardial organs are connected with the c.n.s. via longitudinal 

 trunks (1,2) (Fig. 3.20, p. 118). 



The notion of a secretory function in specialized nerve cells was proposed 

 by Gaskell for so-called chromaffin cells, which are believed to secrete 

 adrenaline. Among lower chordates (cyclostomes, fishes) chromaffin tissue 

 is found in the suprarenal bodies, which contain glandular cells akin to 

 post-ganglionic neurones, and which are innervated by the sympathetic 

 system (6\a). The suprarenal bodies are endocrine glands, which discharge 

 adrenaline into the blood stream. Chromaffin cells, which have been 

 identified in the nerve cord of the leech, several polychaetes and in the 

 heart of Limulus, are thought to have a similar function. In the annelids. 



