166 INVERTEBRATE PHYSIOLOGY 



secretory substances of the invertebrates. Representatives of most of the 

 invertebrate phyla, from flatworms through protochordates, have been 

 shown to have certain speciaHzed neurons that give histological signs of 

 secretory activity (see E. Scharrer and B. Scharrer, 1954a,b ; B. Scharrer, 

 1955). The products of such cells help to regulate a variety of processes, 

 such as chromatophore activity, reproductive activity, and growth phe- 

 nomena, including molting of arthropods. In fact, most physiological pro- 

 cesses in a group like the Crustacea appear to be under primary or second- 

 ary control of substances released from the nerve endings constituting 

 the "sinus glands." It might be added, at this point, that the discovery of 

 organ Y in the Crustacea by Gabe (1953) and demonstration of its role 

 in molt control in the green crab, Carcinits (Echalier, 1954, 1955), makes 

 it appear probable that some postulated actions of neurosecretory sub- 

 Stances in the Crustacea are actually performed by a hormone from organ 

 Y. This gland, however, may be controlled by a neurosecretory substance. 

 By means of electrophoresis, three different "chromactive" (chromato- 

 phore-activating) substances have been obtained from insects and Crusta- 

 cea (Carlisle, Dupont-Raabe, and Knowles, 1955). In no case has the 

 chemical identity of a neurosecretory substance in an invertebrate yet been 

 determined, although such a step forward would appear imminent. 



Modes of Action of the Neurohormones 



Little is known concerning the basic mechanism of action of the neuro- 

 hormones in either the vertebrates or the invertebrates. There is evidence 

 that acetylcholine and adrenaline act at the surface of some cells to pro- 

 duce changes in ion permeability and in resting potential of the cell mem- 

 brane, but the exact series of chemical and physical events is not known. 

 In this section a few examples will be given of recent progress in our un- 

 derstanding of the modes of action of invertebrate neurohormones. 



It was pointed out earlier that neurohormones appear to be of two main 

 types, the neurohumors, which act mostly at short range and for relatively 

 brief duration, and the neurosecretory substances, which may act at some 

 distance from the point of release and for relatively long periods of time. 

 Acetylcholine normally has a very short life after leaving a neuron, be- 

 cause of the abundance of cholinesterase waiting to hyrolyze it. The rate 

 of destruction of the catechol and indole amines when injected or applied 

 is often slower than the destruction of acetylcholine, but amine oxidase is 

 of common occurrence in the invertebrates (Blaschko, 1952). Acetyl- 

 choline and certain of the amines act as transmitters of processes where 

 rapid onset of effect and rapid recovery are required. Often they have 

 opposing actions ; nowhere is this better illustrated than in many mollusc 

 hearts, where acetylcholine has a depressor action while 5-hydroxytryp- 



