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 gill-_slits_ are supported internally by the gill-_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 arrangement is developed from the anterior part of the intestine--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. [Illustration: FIG. 48.--_Balanoglossus_. (After A. Agassiz.) _r_, proboscis; _h_, collar; _k_, gill-slits; _d_, digestive posterior intestine; _v_, intestinal vessel; _a_, anus.] [Illustration: FIG. 49.--A large Sea-lamprey (_Petromyzon marinus_), much reduced in size. (After Cuvier and Häckel.) A series of seven gill-slits are visible.] [Illustration: FIG. 50--Adult Shark (_Carcharias melanopterus_). (After Cuvier and Häckel.)] [Illustration: FIG. 51.--Diagram of heart and gill-arches of a fish. (After Owen.)] [Illustration: FIG. 52.--One gill-arch, with branchial fringe attached. (After Owen.) H, Heart.] [Illustration: FIG. 53.--Diagram of heart and gill-arches in a lizard. (After Owen.) The gill-arches, _a a' a''_, and _b b' b''_, are called aortic arches in air-breathing vertebrata.]