I04 THE EVOLUTION OF MAN. 



much simpler and more imperfect, as, for instance, in the 

 structure of the heart, the circulatory system, and the 

 kidneys. In them, as in the Amphioxus, the anterior 

 portion of the intestinal canal does, indeed, form respiratory 

 gills ; but these respiratory organs are developed in a very 

 peculiar way : in the form of six or seven little pouches, or 

 sacs, which lie on both sides of the anterior intestine and 

 communicate with the throat {pharynx) by inner openings, 

 and by outer ones with the external skin. This, is a very 

 peculiar formation of the respiratory organs, quite cha- 

 racteristic of this class of animals. They have therefore 

 been called the " Pouch-gills " (Marsupobranchii). The 

 absence of one very important organ found in the Fishes, 

 the swimming-bladder, from which the lungs of the higher 

 Vertebrates have developed, should be especially noticed. 



In their germ-history, as in their whole anatomical struc- 

 ture, the Round-mouths present many peculiarities. They 

 are even peculiar in the unequal cleavage of the egg, which 

 most nearly approaches that of the Amphibians (Fig. 31, 

 vol. i. p. 203). This results in the formation of a Hood- 

 gastrula, like that of Amphibians (Plate II. Fig. 11). From 

 this develops a very simple organized larval form, which is 

 closely allied to the Amphioxus, and which, for that reason, 

 we examined and compared with the latter (vol. i. p. 428, 

 and Plate VIII. Fig. 16). The gradual germ-evolution of 

 these larvfe of the Round-mouths explains very clearly and 

 •unmistakably the gradual evolution of the Skulled from the 

 Skull-less class of Vertebrates. At a later period, from 

 this simple Lamprey larva is developed a blind and tooth- 

 less larval form, which is so very different from the mature 

 Lamprey that, until twenty years ago, it was generally 



