INTEGUMENT. 25 



of Merkel, formed by the groupiHg together of 

 terminal ganglion-oells. These sense-orgaus are found 

 principally on the limbs and flanks of Eaiut cscnlenta, 

 more developed in males than in females ; the small 

 warts on the fore limbs of Pelobates, which have by 

 some been compared with the nuptial horny excres- 

 cences, seem to be of the same nature. Very similar 

 productions appear during the breeding season in the 

 females of Bana temporarla and E. arvalis, but are of a 

 merely temporary character, and will be noticed in 

 the chapter on Pairing and Oviposition, together with 

 the horny excrescences which arm the digits or other 

 parts in the males at the same period, and assist them 

 in maintaining their hold whilst pairing. 



Pigment, usually confined to the cutis, sometimes 

 occurs in small quantity in the epidermis. As Leydig 

 has shown, five kinds of special cells may be pre- 

 sent in the cutis — black, yellow, red, white, and 

 metallic or iridescent, the last being the so-called 

 guanin-cells or iridocytes. The yellow and black pig- 

 ments combine to produce the bright green colour, as 

 in normal specimens of Bana escidenta typica and 

 Hyla arborea : if the yellow pigment be absent, as in the 

 Grerman specimens of Bana esculenta, var. ridibunda, 

 the black and gold produce the dull green or olive hue; 

 if both yellow and gold pigments are absent, the black 

 below the cloudy medium of connective tissue and 

 epidermis produces blue, as in the sky-blue specimens 

 of Bana esculenta and EyJa arborea. In the play of 

 the special pigment-cells or chromatophores, which 

 contract or expand and radiate, we find the explana- 

 tion of the changes of colour which some specimens 

 undergo with so great rapidity. A tree-frog will turn 

 from yellow to green and black in less than an hour, 

 yellow when the black pigment contracts, black in the 

 opposite process. 



That these rapid changes of colour harmonise, 

 within certain limits, with the surroundings is well 

 known. The researches of Dutartre on Bana escu- 



