CHAPTER 4 



KINETIC HORMONES 



11. CONTROL OF EXOCRINE AND 

 ENDOCRINE GLANDS 



The hormones to be considered in this chapter all act upon glands ; 

 most stimulate secretion, though a few inhibit it. They fall into 

 two distinct categories; those that stimulate exocrine glands to 

 secrete to the exterior, or into the lumen of some organ such as the 

 gut or a genital duct (§4.1), and those endocrinokinetic hormones 

 that stimulate endocrine glands to secrete their hormones into the 

 blood (§ 4.2). There seems to be every reason for considering the 

 first to be kinetic, like those acting on other effector organs (e.g. 

 the chromactivating hormones, § 3.2), and therefore for classifying 

 the second in the same way (§ 4.2). Yet endocrinokinetic hormones 

 may well be regarded as a group somewhat apart from those acting 

 directly upon other effectors, since their control of bodily functions 

 is not direct but effected through a chain of two hormones. 



4.1 Exocrine glands 



Some exocrine glands secrete continuously; but in many the 

 flow is intermittent, and varied to suit the needs of the organism. 

 The variable glands may, like some of the melanophores, be either 

 under purely nervous control, or purely hormonal control, or even 

 under a combination of the two ; but, as far as is yet known, it is 

 only in vertebrates that hormones are concerned. There are, 

 however, some invertebrates where hormonal control of exocrine 

 glands might be expected and others where it has been reported 

 on insuflicient evidence. 



MoLLUSCA. Intermittent secretion of digestive enzymes is of no 

 advantage to continuous feeders, like the microphagous Lamelli- 

 branchia; but it may be of importance in large carnivorous 



