380 METAZOAN PHYLA 



still contained in the archenteron. About the time the yolk is used up 

 a connection appears between the cavity of the stomodeum and that of 

 the anterior end of the archenteron and another between the cavity 

 of the proctodeum and that of the posterior end of the archenteron, 

 which thus becomes converted into an aUmentary canal. The animal 

 now begins to swim, feeds on algae and other vegetable matter, and is 

 known as a tadpole. At this time the external gills begin to shorten, 

 and internal gills, of which there are four pairs, are being formed. A 

 fold grows around the body just behind the head, covering the gill 

 slits on both sides and producing a chamber known as a branchial pouch. 

 The branchial pouch opens on the left side by a circular opening called 

 the spiracle. By this time the external gills have ceased to function 

 and the internal gills serve in respiration, water being passed through 

 the mouth, on through the gill slits, into the branchial pouch, and out 

 through the spiracle. 



Of the two pairs of limbs the hind pair appear first. Later the fore- 

 limb on the left side emerges through the spiracle, while the one on the 

 right side breaks through the wall of the branchial pouch. The tail 

 diminishes in size, being in part absorbed by cells in the body and in 

 part inclosed by the body, until it is no longer apparent from the outside. 

 The internal gills are also absorbed and lungs develop to function as 

 respiratory organs, after which the gill slits close and the branchial 

 pouch disappears. During the time between the giving up of branchial 

 respiration and the functioning of the lungs the skin becomes very 

 vascular and respiration is carried on through it. Thus metamorphosis 

 takes place gradually and changes the tadpole into a frog differing from 

 the adult only in size. 



403. Neoteny and Pedogenesis. — Pedogenesis has already been 

 defined as the production of young by an immature animal. In the 

 tailed amphibians cases are known in which the larval characters are 

 retained until after sexual maturity. These animals may either be 

 looked upon as adults which, having not metamorphosed, retain certain 

 larval characteristics, or they may be considered as being larvae in 

 which the reproductive organs are precociously developed. This 

 prolongation of larval characteristics into advanced age has been termed 

 neoteny, and reproduction by these animals may be termed pedogenesis. 

 A classical example of these is seen in the larvae of species of Ambystoma. 

 Under certain circumstances these salamanders do not metamorphose 

 but retain their gills and their aquatic life and yet become sexually 

 mature. In this condition they are known as axolotls (Fig. 2Q0B). 

 It has been found possible under experimental conditions to control 

 metamorphosis and to produce the axolotl type at will. In nature these 

 are particularly abundant in alkaline lakes and ponds throughout the 

 semiarid regions of the West, and south into Mexico. 



