ORGANIC FORM AND ARCHITECTURE 689 



in which there is also dwindhng of size and sometimes a general 

 simphfying of structure. The subject will be referred to again in 

 connection with Evolution and Heredity. 



CLASSIFICATION OF ORGANS— The most natural arrange- 

 ment of organs is not morphological, but physiological. They may 

 be arranged according to the part they play in the life of the animal. 

 Thus some have to do with external relations, and may be arranged 

 as locomotor, prehensile, food-receiving, protective, aggressive, 

 courting, and mating organs. Of internal organs, the skeletal 

 structures are passive, compared with active organs like the heart, 

 or the gizzard. Some are alimentary, others nervous, sensory, 

 glandular, vascular, and so on ; and a special rank must be ascribed 

 to the regulatory, hormone-making organs, like the thyroids and 

 the suprarenals. The essential reproductive organs or gonads, 



Fig. ioi. 



Rattle of Rattlesnake, Crotalus horridus. From a specimen. This instrument 

 is composed of a series of horny rings, added to at each sloughing, but 

 continually dropping off terminally. A very rapid vibration produces a 

 shrill whistle-like sound, which serves to warn off intruders too large to 

 be of use. In the presence of a small rodent the rattling ceases. The 

 rattlesnake does not hiss. 



though also hormonic in Vertebrates, deserve to be placed by 

 themselves. 



But it is also of interest to classify organs according to the 

 embryonic layer or layers from which they arise. Thus to the 

 ectoderm must be referred all epidermic structures, such as hoofs 

 and claws, all nervous organs, and much at least of all the sense- 

 organs. The inner layer or endoderm forms at least an important 

 part (the "mid-gut") of the food-canal, and the basis of all its 

 outgrowths — lungs, liver, pancreas, and the like. The skeletal 

 notochord is also endodermic. To the middle layer or mesoderm 

 of the embryo must be referred the endoskeleton (except the noto- 

 chord), the muscles, the connective swathings, the heart, the 

 kidneys, the spleen, and so on. In many cases there is a dual origin; 

 thus, as abready stated, every outgrowth from the Vertebrate 

 gut arises as an endodermic pouch, but carries the enswathmg 

 mesoderm out with it. The eye is essentially an ectodermic organ, 

 but the mesoderm forms the firm outer layer or sclerotic, as well as 

 the internal vitreous humour. Many relatively simple superficial 

 structures such as horns are partly ectodermic and partly meso- 



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