GENERAL ZOOLOGY 



not suffice, however, for more complex animals leading more active lives. 

 Even among higher protozoans, as in Paramecium, protoplasmic organelles 

 have developed to serve in coordinating the activities of different parts of the 

 unicellular body. The problem of adequate integration is more acute in 

 multicellular forms, which are generally much larger. Here an environ- 

 mental change or stimulus acting upon some part of the body must be reacted 

 to bv the body as a whole, or by some part distant from the point of stimula- 

 tion; and the reaction may commonly involve thousands of cells. As we have 

 seen, in most metazoans, special cells, tissues, and organs, culminating in 

 what may be termed sensory-neuro-motor systems, have evolved as integrative 

 mechanisms. These represent devices by which animals rapidly perceive en- 

 vironmental changes, and by which appropriate responses are quicklv .set in 

 motion. 



In Chapter 4, an account was presented of another type of coordinating 

 mechanism as it appears in vertebrates, involving endocrine organs and 

 their secretions, the hormones. For the most part, these are adapted to 

 slow, long-term integration of bodily changes, such as regulation of growth, 

 control of metabolic rates, cyclic maturation of gonads and gametes, and so 

 on. It was long supposed that this chemical coordination was limited to 

 vertebrates, but a steadily growing body of information makes it clear that 

 analogous mechanisms operate in a wide variety of invertebrates as well. 

 Here we shall present a brief survey of the general features of these dual 

 integrative systems, nervous and chemical, as they occur in some typical 

 invertebrates. 



Sensory-neuro-motor Systems. The behavior of protozoans reveals 

 that they respond to environmental changes in predictable ways. Par- 

 ticularly among ciliates (pp. 2.50-253), it is possible to find, in the sensory 

 cilia, the conductile fibrillar systems, and the contractile motor organelles, 

 intracellular structures which parallel the mechanisms for reception, con- 

 duction, and response in multicellular forms. 



In the Porifera the only normal reactions to stimuli appear to involve the 

 closing of pores and oscula, contractions of the entire sponge, and adjustment 

 of the diameters of parts of the canal systems. The effectors for these 

 activities are simple, spindle-shaped muscle cells arranged about openings 

 and canals. These were long believed to be independent eflFectors, but the 

 recent discovery of typical though primitive nervous elements in sponges 

 makes it appear that these organisms possess well-developed receptor-effector 

 mechanisms of coordination. Neurons apparently conduct impulses from 

 choanocytes and from simple surface receptors to the muscle cells and mediate 

 coordinated activities of a simple kind throughout the body of the sponge 

 (Fig. 17.7). 



The nervous system of coelenterates is of the type described for the hydra 

 (pp. 293-297). Sensory cells and neurosensory cells are the receptors,- spe- 

 cialized nerve cells perform the function of conduction; and the contractile 

 processes of the large epitheliomuscular cells are effectors. These relationships 



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