AMPHIBIA 205 



proximally and distally, the remaining two small elements of 

 the tarsus, the five metatarsalia, and the phalanges. The 

 phalanges number 2, 2, 3, 4, 3. There is in addition, on the 

 medial aspect of the first toe, a small process called the calcar. 

 This consists of a small metatarsal and one or two phalanges, 

 and it is believed to be a prehallux, or additional toe, which 

 has early in the phylogenesis of the Amphibia and their 

 successors been discarded. 



The skin is thin and tends to become resolved into a super- 

 ficial horny layer of flat cells and a lower layer based on a 

 Malpighian layer. The former is moulted periodically, the 

 shedding beginning at metamorphosis and continuing during 

 growth. The discarded stratum corneum is eaten. The 



N.sp.. N.can N.SI 

 Art.proc. \ / \ 

 Tr.proa I / Ischium 



Centrum 



Vrostyle 



Acetabulum" 



Pulis 



FIG. 104. Pelvic girdle and posterior (vertebrae from the side. 

 An endjview of a vertebra is given. 



skin is beset with mucous glands, and in some frogs a special 

 accumulation of glands is formed on each side dorsally, and 

 these are called paratoid glands. The secretion of the para- 

 toid glands of toads, newts, and salamanders is poisonous. 

 The ectoderm is supported on a connective- tissue dermis 

 bearing blood-vessels and nerves, and it is into this layer 

 that the skin glands grow from the ectoderm. The skin, 

 moreover, is to a large extent separated from the muscular 

 wall of the body by large spaces containing lymph. 



The alimentary canal is formed almost entirely of endo- 

 derm. The anterior ectodermal invagination, or stomodeum, 

 contributes the front part of the mouth, and the posterior 

 invagination, or proctodeum, the posterior part of the cloaca. 



The general features of the mouth cavity have already 

 been described. The mucous membrane of the mouth narrows 



