NEUROHORMONES 163 



scribe these vesicles in the neuropile of the earthworm nerve cord as oval 

 or spherical bodies between 200 and 400 A in diameter, with a dense 

 periphery and a lighter center. They find them in large numbers in pre- 

 synaptic terminals and essentially lacking in postsynaptic processes. They 

 also find them apparently lying between pre- and postsynaptic mem- 

 branes and suggest that the vesicle may penetrate the presynaptic mem- 

 brane and discharge its contents or possibly enter the postsynaptic 

 cytoplasm. 



There has been a growing body of evidence that acetylcholine, adrena- 

 line, and the neurohormones of the posterior pituitary are contained in 

 cell particles with many of the properties of mitochondria. Some of this 

 evidence has been summarized elsewhere (Welsh, 1955). It would now 

 appear that these mitochondria-like particles are the "vesicles" and 

 "granules" now seen in a variety of nerve endings. It remains to be de- 

 termined whether the "spheroid systems" seen by Passano (1953) in the 

 cell body, axons, and terminals of neurosecretory cells of the marsh crab, 

 Sesarma reticulata, are aggregates of smaller vesicles which are formed 

 in the cell body and move down the axon for storage in the axon terminals. 



Crustacean sinus glands, long thought to be more or less typical en- 

 docrine organs, were first recognized to be groups of nerve endings by 

 Passano (1951) and Bliss (1951). These endings are modified for stor- 

 age and release of neurosecretory substances into the circulation. Other 

 examples of nerve endings in crustaceans that release substances directly 

 into the circulation are the pericardial organs described by Alexandro- 

 wicz (1953) and the "sinus plates" of the prawn, Lcandcr, described by 

 Knowles (1953). The pericardial organs are neuropile-like networks of 

 nerve fibers that spread across the openings through which blood enters 

 the pericardial cavity. They release a heart-accelerating substance into the 

 blood which is one of the means of regulating the heart rate ( Alexandro- 

 wicz and Carlisle, 1953). Sinus plates consist of nerve endings which 

 contain a chromatophore-activating substance (Knowles, 1953). Carlisle 

 and Knowles (1953) suggested that groups of nerve endings that do not 

 innervate a structure but instead store and release active materials di- 

 rectly into the circulation, be called "neurohaemal organs." In the insects 

 the corpora cardiaca are regions where most of the axons from neuro- 

 secretory cells in the brain terminate and release their neurosecretory 

 substance (Scharrer and Scharrer, 1944, 1954b). Therefore, "sinus 

 glands," corpora cardiaca, pericardial organs, and the vertebrate posterior 

 pituitary, all analogous structures, are examples of neurohaemal organs. 

 Neurohumors, such as acetylcholine and nor-adrenaline, are probably 

 stored and released in a manner similar to that described for the neuro- 

 secretory substances. Some of the evidence to support this view, gained 



