COORDINATION AND BEHAVIOR 



551 



spects. With the aid of the lens, an image 

 of the objects in front of the cornea is 

 formed on the sensitive retina as on photo- 

 graphic film. The eye is accommodated for 

 recording images of near and far objects by 

 changes in the convexity of the lens. These 

 changes in the lens are caused by its own 

 elasticity, and by the pull exerted upon it by 

 the elastic choroid coat and the ciliary mus- 

 cles. In viewing near objects, the ciliary 

 muscle counteracts the pull of the choroid 

 coat and allows the lens to assume a more 

 convex shape, whereas distant objects are 

 made distinct by the flattening of the lens. 



The eye is moved by 6 muscles: 4 straight 

 (rectus) and 2 oblique. Folds of skin, the 

 eyelids, protect the eye in higher vertebrates. 

 In the vertebrate there may be three eye- 

 lids: an upper and a lower lid which act 

 vertically, and a lateral lid (nictitating mem- 

 brane) which moves outward from the inner 

 angle of the eye. In some reptiles the eyelids 

 are transparent and fused over the eye. 

 Terrestrial vertebrates have lacrimal glands 

 in connection with the eye; the secretion 

 from these keeps the surface of the eyeball 

 moist and washes away foreign particles. 



HORMONES; CHEMICAL 

 COORDINATORS 



We have seen how the nervous system 

 coordinates the separate responses of an 

 animal, as for example, coordination of 

 muscles in writing, talking, and walking. 

 These activities involve the working to- 

 gether of various sets of muscles, and the 

 nervous system is very effective in quickly 

 correlating and regulating these activities. 

 In contrast, the endocrine system usually 

 acts slowly over long periods of time in 

 effecting coordination. The endocrine sys- 

 tem is composed of specialized tissues, and, 

 in some cases, glands, which produce chem- 

 ical substances called internal secretions 

 (hormones). Most multicellular glands in 

 animals have ducts and are called glands of 



external secretion (exocrine); an example 

 is the liver which discharges its secretion 

 through a duct into the intestine. The 

 endocrine glands, however, are ductless, and 

 hence are called glands of internal secretion 

 because their hormones pass directly into 

 the blood or other body fluids. A hormone 

 is a chemical coordinator which has some 

 regulatory effect upon cells that may be 

 some distance from the origin of the hor- 

 mone. A hormone may be either excitatory 

 or inhibitory in its influence on the activities 

 of various tissues and the behavior of an 

 animal. 



Hormones of invertebrates 



Most of the research on hormones has 

 been done on vertebrates. As a result, com- 

 paratively little is known about hormones 

 in the different invertebrate phyla. There is 

 good evidence of hormones in nematodes, 

 annelids, arthropods, mollusks, and some 

 other invertebrates. Hormones in certain of 

 the arthropods are discussed in an earlier 

 chapter. Destruction of certain gonadal 

 tissue of some crabs often results in partial 

 sex reversal, somewhat similar to that ob- 

 served in chickens. In the bug Rhodnius, 

 the hormone controlling molting is secreted 

 by glands in the head. Molting and meta- 

 morphosis in certain arthropods are under 

 the control of hormones. 



Hormones of man 



In striking contrast to the situation among 

 invertebrates, in which little is known about 

 endocrine function, a great many investiga- 

 tions have been conducted on many verte- 

 brates. The methods used have been either 

 to remove an endocrine gland or to study 

 the effects of injected hormones. Additional 

 information has been obtained from research 

 on animals in which the endocrine glands 

 were diseased. Some experiments have been 

 performed on the effects of transplanting 

 glands. Several of the hormones such as 



