GILL-SLITS 285 



because no other animals besides vertebrates possess such 

 a spinal rod. 



We have seen that the frog's and the Ascidian's tadpole 

 alike possess the " notochord." The next point of 

 resemblance is that both tadpoles have on each side of 

 the gullet a pair of slits (becoming more numerous in 

 later stages of growth), which allow the water, taken in by 

 the mouth, to stream out again on each side (see Fig. 35). 

 All vertebrates (even man and monkeys) have in very 

 early life these " gill-slits." They remain permanently in 

 fishes as the breathing organs or gills, and so they do 



POSITION OE SPIBACLE 



'CHORD / | QRAIN MOUTH 



MOTOCHORD 



'ClU SLITS 



FIG. 35. A diagram of the anatomy of the tadpoles of the frog (upper 

 figure) and of the Ascidian (lower figure). The notochord or 

 skeletal spinal rod, the gill slits, and the brain and spinal cord are 

 seen in each. 



in the Ascidian in the form of multiplied perforations 

 of the gullet, as we have seen (Fig. 32). But no other 

 animals besides vertebrates have these slits. In the 

 frog's tadpole and in that of the Ascidian the body- 

 wall grows out over the gill-slits, so as to form a 

 protective chamber, the peri-branchial chamber or gill 

 chamber, with a spout-like pore, to let the water stream 

 away, but in the frog's tadpole the terminal opening of 

 the intestine is not covered in by the walls of this cham- 

 ber as it is in the Ascidian's tadpole and in the adult 



