446 



THE BIOLOGY OF MARINE ANIMALS 



at least, adrenaline is believed to originate in the cell body, from which it 

 is transported along the axon to be released at the terminals. 



The sinus gland of crustaceans resembles the posterior pituitary of 

 vertebrates in being a storage-release centre for products produced else- 

 where (in the x-organ and other central nervous secretory centres of 

 crustaceans, in the hypothalamus of vertebrates). The cytological appear- 

 ance of the secretory neurones in crustaceans suggests that secretory 

 material is formed in the cell body, is transported along the axon, and 

 stored in the sinus gland. Under the influence of impulses from the neuro- 



Fig. 10.20. Neurosecretory Systems in the Land Crab Gecarcinus lateralis 



{Left) Dorsal view of the brain; {right) anterior view of the right eye-stalk. Black areas 

 {B\ — B5, El, E2, EA) indicate locations of neurosecretory cells. Double lines indicate some 

 fibre-tracts to neurosecretory cells. BST, brain/sinus gland tract; CC, circumoesophageal 

 (tritocerebral) connective; LG, lamina ganglionaris ; ME, medulla externa; MI, medulla 

 interna; MT, medulla terminalis; PLO, optic lobe peduncle; SG, sinus gland; SGT, 

 sinus-gland tract; XST, x-organ/sinus-gland tract. (From Bliss et al. (11).) 



secretory cell bodies, the storage products are released from the sinus 

 gland in activated form as hormones. 



Among the known regulatory functions of the sinus gland/x-organ 

 neurosecretory system of Crustacea are: acceleration and inhibition of 

 moult; adjustment of respiratory levels; calcium deposition; control of 

 chromatophore activity and retinal pigment migration. The triggering 

 factor for hormonal release may be external, as in the case of chromato- 

 phore responses, in which afferent impulses from the retina enter the c.n.s. 

 and eventually reach the neurosecretory centres. In many other instances 

 we must postulate internal releasing agencies for functional activities such 

 as onset of moult, metabolic rhythmicity, etc. Extracts of the pericardial 



