PROCHORDATA AND CYCLOSTOMES 321 



clefts, between the branchial arches. Animals cannot 

 break up the molecule of water (H 2 O) and take the 

 oxygen; -they have to depend on the small amount of 

 that gas which is dissolved in the water. Consequently, 

 if the breathing apparatus is not very adequate, they 

 may have to live near the surface or in running water. 

 Various insect larvae with external gills, which live in 

 running streams, perish from suffocation if placed in 

 a dish of still water. Now the gill-cleft arrangement 

 is one for creating a stream, which flows continually 

 past delicate tissues full of blood, which are at the same 

 time largely concealed and protected from injury. It 

 is evidently an advance in mechanical organization, - 

 an invention of Nature, as it were. 



2. These being the more fundamental characters of The 

 the vertebrates, we naturally ask ourselves, whence did 

 they come ? Are they wholly peculiar to these animals ? 

 Seeking an answer to this question, we come upon a 

 series of animals which certainly are not vertebrates, 

 because they possess no vertebral column ; yet they 

 possess, in greater or less degree, the notochord, the 

 dorsal nerve cord, and the gill-slit apparatus. These 

 creatures belong to entirely distinct groups, typified 

 by the Amphioxus, the Balanoglossus, and the tunicate 

 or sea squirt. All are marine, and of comparatively 

 small size. This series of animals, thus set apart from 

 the vertebrates and invertebrates alike, is grouped under 

 the name Prochordata, mainly as a matter of conven- 

 ience. It is not certain that some of the characters 

 mentioned may not be found or have existed among the . 

 invertebrates ; thus Professor Patten of Dartmouth 

 College describes a notochord as existing in a scorpion. 

 In his opinion the scorpions (a very ancient group, 

 certainly) are the survivors of the gigantic Eurypterids 



