54 The Vertebrate Organ Systems 



anterior end. The posterior end is free and notched, and is rapidly 

 extended to catch insects. This end is coated with mucus which aids 

 in holding any captured prey. Taste buds are scattered over the sur- 

 face of the tongue. 



Behind the tongue, in the pharynx, is a median ventral slit, the 

 glottis. The glottis opens into the larynx which in turn leads into the 

 lungs. The vocal cords are located in the larynx. On either side of 

 the glottis are fleshy folds which close when food is being swallowed, 

 but open to let air into the lungs during respiration. Posteriorly the 

 pharynx narrows and continues as the esophagus of the digestive tract. 



THE INTEGUMENTARY SYSTEM 



The skin of the frog unlike that of many other vertebrates is only 

 loosely attached in a few areas to the underlying tissues. It is quite 

 thin and must be kept moist as the frog relies upon it for much of his 

 necessary respiratory exchange. A rich supply of blood vessels makes 

 possible this dermal respiration. 



Histologically the skin consists of two main divisions : the outer 

 epidermis and the inner dermis or corium. The epidermis consists of 

 two main layers. The outer one is formed of flattened squamous epi- 

 thelial cells and is called the stratum cornemn. Below this is a transi- 

 tional layer consisting of cuboidal cells. This layer in turn merges 

 into the innermost layer, the stratum germinativum, which is formed of 

 columnar epithelial cells. Periodically the outermost layer is sloughed 

 ofif. The cells of the stratum germinativum are continuously dividing 

 to replace this outer layer. 



The dermis likewise consists of two main layers. The outer one, 

 the stratum spongiosum, is composed of loose connective tissue con- 

 taining pigment cells and various glands. The connective tissue of 

 the inner layer, the stratum compactum, is densely packed and contains 

 blood vessels, nerves, and some muscle cells. 



To a great extent the skin of the frog is kept moist by numerous 

 small mucous glands which are located in the dermis and which open 

 to the outside through short ducts. Also located in the dermis are a 

 small number of large poison glands that are able to discharge an irri- 

 tating secretion. All the glands are fiask-shaped and have muscle fibers 

 at their bases for forcing out the secretions. The mucous glands which 

 help to keep the frog slippery and the poison glands which make him 

 distasteful undoubtedly are of aid in helping this animal to slip from 

 the grasp of would-be diners. 



