48 



THE FROG. PHYLUM CHORDATA 



being absent. The first finger of the male frog bears at the breeding 

 season a rough-skinned swelhng, not unHke the ball of the human 

 thumb (Fig. 261). The ankle is much longer than the wrist, and 

 all five toes are present and united by webs of skin, so that a 

 wide surface is provided for use in swimming. 



Between the legs at the hinder or posterior end of the trunk is 

 the vent or cloacal opening, through which are passed the faeces, 

 urine, and eggs or sperms. 



SKIN 



The skin of the frog is a thin, tough, protective covering. 

 The dermis or inner layer contains pigment cells (Figs. 262, 263 B), 

 and embedded epidermal glands of several kinds which between 



—Connective tissues of the frog. 



A, Tendon ; B, pigment cells in the skin, seen through the epidermis. 



c, Cells ; p.con., pigment cell with the pigment contracted ; p.ex., pigment cell with the pigment 



extending into the processes ; w.f.. white fibres. 



