Embryology. 147 



arrangement is permanent. It is likewise met with in a 

 peculiar kind of worm, called Balanoglossus a creature 

 so peculiar, indeed, that it has been constituted by 

 Gegenbaur a class all by itself. We can see by the 

 wood-cuts that it presents a series of gill-slits, like the 

 homologous parts of the fishes with which it is compared 

 i.e. fishes of a comparatively low type of organization, 

 which dates from a time before the development of 

 external gills. (Figs. 48, 49, 50.) Now, as I have 

 already said, these g\\\-slits are supported internally by 

 the g\\\-arches, or the blood-vessels which convey the 

 blood to be oxygenized in the branchial apparatus 

 (see below, Figs. 51, 52, 53) ; and the whole arrange- 

 ment is developed from the anterior part of the in- 

 testine as is likewise the respiratory mechanism 

 of all the gill-breathing Vertebrata. That so close 

 a parallel to this peculiar mechanism should be met 

 with in a worm, is a strong additional piece of evidence 

 pointing to the derivation of the Vertebrata from the 

 Vermes. 



Well, I have just said that in all the gill-breathing 

 Vertebrata, this mechanism of gill-slits and vascular 

 gill-arches in the front part of the intestinal tract is 

 permanent. But in the air-breathing Vertebrata such 

 an arrangement would obviously be of no use. Con- 

 sequently, the gill-slits in the sides of the neck (see 

 Figs. 16 and 57, 58), and the gill-arches of the large 

 blood-vessels (Figs. 54, 55, 56), are here exhibited 

 only as transitory phases of development. But as 

 such they occur in all air-breathing Vertebrata. And, 

 as if to make the homologies as striking as possible, 

 at the time when the gill-slits and the gill-arches are 

 developed in the embryonic young of air-breathing 



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