768 Comparative Animal Physiology 



and cell differentiation), those processes in which the controlling influence 

 needs to be maintained over long periods of time (as in the control of gen- 

 eral processes in basic maintenance and metabolism), and situations in 

 which effector organs are maintained in one or another condition for very 

 long periods of time (as with chromatophores). 



Within even relatively large natural groups of animals there is commonly 

 a lack of specificity of the hormonal substances. Among the vertebrates, hor- 

 mones such as the gonadal hormones, insuHn, thyroxin, and others seem 

 rather widely distributed, typically producing within any species a type of 

 response characteristic for that species. Similarly, among the insects, the 

 juvenile hormone and the GD hormone (see p. 733) appear interchangeable 

 among the various species, and often even among orders. The same situation 

 appears also to obtain for the chromatophorotropins, retinal pigment hor- 

 mone, and molt-inhibiting hormone among various species of the Crustacea. 



Beyond these groups little is known as to chemical or physiological simi- 

 larities of the active principles. The corpora cardiaca of insects yield a prin- 

 ciple highly active on the chromatophores of crustaceans. A rather extensive 

 literature has developed regarding the influence of invertebrate hormones 

 on vertebrates and especially of vertebrate hormones on invertebrates.'^^- ^^^ 

 Much of the work is confusing and contradictory and gives us little reason 

 for believing that the results have anything other than interesting pharma- 

 cological value. The vertebrate chromatophorotropin, intermedin, and the 

 crustacean hormone, UDH (see p. 698), appear qualitatively to resemble 

 one another in their chromatophorotropic action and in many of their physi- 

 cochemical properties.^ A critical examination, however, gives us strong 

 reason for believing they are not identical.^"^ 



The nervous systems of a wide variety of animals show histological and, 

 in certain cases, good physiological evidence of the differentiation of en- 

 docrine elements or neurosecretory cells. ^^^ Such cells occur in certain por- 

 tions of the nervous systems of worms, molluscs, arthropods, and vertebrates. 

 In the last group, they are located in the nucleus preopticus of fishes and am- 

 phibians, and in the homologous nuclei supraopticus and paraventricularis 

 of reptiles and mammals. Only among certain invertebrates is there evidence 

 of a functional role of these neurosecretory cells within the organism. The 

 pars intercerebralis of the insect brain contains knots of large neurosecretory 

 cells and the endocrine activity of this part of the nervous system has al- 

 ready been described. A certain chromatophorotropically active agent for 

 crustacean chromatophores has been found to have a quantitative distribu- 

 tion in the nervous system of Limulus closely paralleling the frequency of 

 neurosecretory cells known to occur there. ^'*' ^'^^ Certain large ganglion cells 

 in annelids give a chromaffin-staining reaction characteristic of adrenalin- 

 producing tissues, and extraction and assay of such nervous tissue gives posi- 

 tive physiological tests for adrenalin. 



It is interesting to point out that the insect corpora cardiaca and allata are 

 so closely associated with the nervous system that they were formerly known 

 respectively as the anterior and posterior esophageal ganglia; the corpora 

 cardiaca contain ganglion as well as secretory cells. The sinus gland of crus- 

 taceans appears to be a derivative of the neurilemma! sheath of the anterior 

 central nervous system, as does also the glandular source of the hormones of 



