STRUCTURE AND FUNCTION IN INVERTEBRATES 



ment of one-way conduction and of reflex circuits, in place of the diffuse 

 nerve pathways characteristic of the net-like system of sponges and coelen- 

 terates. The subepidermal plexus of annelids, a minor part of the nervous 

 system, is similar in structure and function to the nerve net of coelenterates. 

 The brain of the annelid may be interpreted as an adaptation to its active, 

 progressive wav of life, correlated with the development of sense organs in 

 the anterior region of the body. 



In Mollusca and Arthropoda the nervous system is a receptor-adjustor- 

 efTector mechanism essentially like that of annelids. Active and highly 

 evolved forms, such as squids among mollusks and many insects and spiders 

 among arthropods, have specialized sense organs and effectors of great com- 

 plexity, and the adjustor mechanisms are correspondingly complicated. In 

 the squid, particularly, large areas of the brain are given over to purely 

 associative functions. The same situation exists in connection with the 

 mechanisms of nervous coordination in vertebrates. Thus, from the earth- 

 worm to man the differences consist in the increased number and specializa- 

 tion of receptors, adjustors, and effectors, not in changes of basic cellular 

 relationships. It is very interesting to realize that, in invertebrates as 

 well as vertebrates, the passage of a nerve impulse from one neuron to an- 

 other, or to a motor end organ, appears commonly to involve the liberation of 

 a chemical mediator. This secretion, if released in adequate quantities within 

 a sufficiently short period of time, stimulates the second neuron or the effector; 

 the mediator is prevented from exerting its effect continuously by the presence 

 of a specific enzyme which brings about the immediate destruction of the 

 mediator. There are several known substances capable of thus controlling the 

 spread of nervous excitation, and some are common to both vertebrates and 

 invertebrates (seep. 113). 



Endocrine Mechanisms. In addition to nervous integration, many 

 invertebrates have evolved mechanisms of chemical coordination analogous 

 to those of vertebrates. Such mechanisms are best known among Arthrop- 

 oda. In crustaceans they are involved in control of integumental color 

 changes and of molting, and in insects they regulate growth, differentiation, 

 and metamorphosis (p. 456). Other hormonal effects, suspected but less 

 definitely established, appear to operate in the expression of sex, in the 

 maintenance of blood-sugar levels, and in the general control of metabolic 

 rates. The endocrine organs of invertebrates are composed largely of 

 neurosecretory cells, which appear to be nerve cells modified to perform 

 special secretory functions (Fig. 17.8). As recalled above, the passage of 

 a nerve impulse from one neuron to another appears always to involve the 

 secretion of a chemical mediator, and to this extent all neurons are neuro- 

 secretory cells. However, the secretions of the specialized endocrine cells 

 of invertebrates are not the same as those involved in the stimulation of 

 adjacent neurons or effectors. Hormones may pass by way of nerves, or 

 they may be released into the blood or body fluids and exert their character- 

 istic effects on other organs or tissues distant from the source of the hormone. 



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