46 



ESSENTIALS OF BIOLOGY 



chambered. It may be associated with lungs, gills, ctenidia, etc., 

 and so on, yet the system of a closed series of channels through 

 which blood is propelled by a heart, is the essential structure. 



great ^^j^^^^nj^^^^ 



joined bu ccLpillaries 



HecLvt Z^ 'Secretort/ 



cells} 



Valu 



^'Vcdve 



Fibres of 

 heart TwuLScle 



Dtu:t-\ I 



jirteri/ 



Vein 



Arteriole 



VeiTL 



Fig. 12. 



1, Diagram of the circulation in a fish; 2, diagram of the simple, one-chambered heart; 

 3, a simple gland ; 4, a racemose gland ; 5, a ductless gland ; 6, blood capillaries ramifying 

 through the cells of a gland. 



126. Glandular Organs. Such are, for instances, salivary 

 gastric, sweat glands, etc., kidneys, adrenal, thyroid, etc. glands. 

 Whatever a gland may be, its essential structure is always this : 

 there are secretory cells among, or outside, which blood-vessels 

 ramify. Sometimes (and in general) the secretory cells enclose 

 a cavity into which the secretion is poured (from the cells). 

 Usually a duct carries away the secretion from the gland. The 

 forms of the glands and ducts vary greatly. Sometimes there is 

 no duct (thyroid) and then the secretion is carried in the blood 

 that feeds the gland. 



12/. Organs of the Nervous System. In the coelenterates 

 the nervous system is a simple plexus, or network of nerves. This 

 primitive nervous system also exists in the walls of the alimentary 

 canal of the highest animals and it can effect complex reflex 

 actions. Usually there is a central, ganglionic mass, or brain, 

 or several such organs. 



There are sense-organs and the essential element of such are 

 the terminations of single nerve-fibres. From such a receptor 

 a fibre passes into a nerve centre and another fibre passes out to 



