668 



COMPARATIVE ANATOMY 



sub-unguis forms the 'sole' inside it. The 'frog' is the reduced ball of 

 the toe which projects into the hoof from behind. 



The comparisons in this semester's work will be between fishes, 

 amphibians, reptiles, birds, and mammals, as these represent the great 

 type-forms of vertebrates. 



FISHES 



The life in water makes horny cornification very rare. The epider- 

 mis of fishes is therefore soft. "Pearl organs," however, appear during 

 the breeding season in some teleosts. Glands are quite abundant, the 

 secretion furnishing the slime on the surface. Some groups of fishes also 

 possess poison glands, usually in close relation to the spines of the fins. 

 The elasmobranchs have large pterygopodial glands in the "claspers" of 

 the males, although what these are for we do not know. 



Photophores are some of the most interesting and striking of all 

 epidermal organs. They are usually found in elasmobranchs and teleosts 



from the deep seas, where 

 sunlight does not penetrate. 

 In reality, they are formed 

 very much like an eye by the 

 cells of the Malpighian layer 

 dipping into the corium. 

 Here they are cut off from 

 their origin, forming a 

 deeper glandular layer, and 

 the outer rounded body 

 called the lens. The corium 

 then forms a reflecting layer, 

 which in turn is enclosed by 

 a coat of pigment. 

 In the myxinoids, there are many thread-cells in little pockets 

 located in various portions of the skin. Each of these cells contains a 

 long thread which is discharged upon stimulation, the threads forming 

 a network in which the mucus secreted by the ordinary gland cells is 

 entangled. Artificial pearls are made from "essence of pearl," which is 

 formed in the fibrous tissue of the corium of some fishes. :;i , 



AMPHIBIA 



The interesting-point about these animals is that during the early 

 larval stage, the epidermis is often ciliated, and two cells in thickness. 

 There are numerous mucus and poison glands, sometimes enlargements 

 of the neck called "parotid glands." These occur on the anura, and there 

 is likewise a gland on the back near the base of the tail. It will be re- 

 membered that the large lymph spaces under the skin of the frog make 

 it possible to remove that animal's skin quite readily. As amphibians 



Fig. 395. 



Section through the scale of a Lizard. 1. Peridermal 

 layer. 2. Heavily cornified cells forming the scale. 3. 

 Pigment cell. 4. Ordinary cells of horny layer. 5. 

 Innermost Malpighian layer. 6. Dermis. (After Ship- 

 ley and MacBride.) 



