134 KINETIC HORMONES — II 



kinetic hormone is stimulating the secretion of an endocrine gland. 

 It will be noticed that, although this is often accompanied by 

 growth of the gland, recent evidence shows that the two actions 

 may be due not to the same hormone but to two separate, though 

 closely similar, hormones. Reduction or inhibition of the secretion 

 of endocrine glands is usually due to an indirect "feed-back" 

 system, whereby the accumulation of the metabolic or morpho- 

 genetic hormone in the blood depresses the output of the 

 endocrinokinetic hormone, which had induced that accumulation 

 (e.g. ACH and ACTH, § 4.231). 



Since these endocrinokinetic hormones stimulate the secretion 

 of a second hormone which nearly always has metabolic or 

 morphogenetic effects, they will be more fully understood after a 

 consideration of the hormones that they control. The reader with 

 little previous knowledge of hormones is therefore recommended 

 to read Chapter 5 on metabolic hormones, before considering 

 their control by endocrinokinetic hormones in the present chapter. 

 Reference could well be made to each case as it arises, to which 

 end the section numbers are given as a guide. An account of the 

 morphogenetic hormones, and more details of the endocrinokinetic 

 hormones which control them, will be given in Part II. 



4.21 ECTODERMAL ENDOCRINE GLANDS OF ARTHROPODA 



Ectodermal glands that are stimulated to secrete by endocrino- 

 kinetic hormones are few; the only glands that have so far been 

 postulated all arise in the heads of Arthropoda (Part II). 



The glands derived from ectodermal invaginations in the anten- 

 nary or 2nd maxillary segments all secrete hormones which have 

 a moult-promoting action. The fact that their secretion can be 

 stimulated by endocrinokinetic hormones has been fully estab- 

 lished in a number of insects, but is still uncertain in crustaceans. 



4.211 Y-organ possibly stimulated by a secretion from Hanstrom's 

 sensory pore organ 



Crustacea. The Y-organ has been identified in many Crustacea 

 and is derived from either the antennary or the 2nd maxillary 

 segment, in whichever position is unoccupied by the excretory 

 organ. Its hormone has both metabolic and morphogenetic actions. 



