SALPA IN RELATION TO EVOLUTION OF LIFE. 187 



When, in my student days, my instructor held before me the 

 skull of a turtle and called upon me to observe the centrum, the 

 transverse processes and the neural arch of the occipital vertebra, I 

 was, for the time, convinced that the occipital bone had arisen by 

 the differentiation and specialization of a bony vertebra, like those 

 in the neck of a turtle, and that its history had been identical with 

 that of the thoracic vertebrae, which have been differentiated and 

 specialized in the same way into constituent parts of the bony box 

 which covers the body of the turtle, as the skull covers the brain. 



In all these cases the morphological resemblance is undeniable, 

 but our opinion of its phylogenetic significance depends upon our 

 view of the nature and origin of the metamerism of vertebrates, a 

 question which will soon be discussed. 



At present we must confine ourselves to a narrower point of view, 

 and learn where we are led by Dohrn's opinion that the vertebrate 

 mouth is actually a pair of gill-slits. 



If the present mouth of the vertebrates was once a pair of gill- 

 slits, the ancestors of the vertebrates must have had at that time 

 another mouth, and during the long series of stages of development, 

 while the gill-slits were gradually assuming the function of a mouth, 

 food must have been taken in through both openings ; for the new 

 function of the gill-slits must have been acquired slowly alongside 

 their old function, until the new mouth finally became so perfectly 

 adapted for its new function that it supplanted and replaced the 

 old one. 



Accordino^ to Dohrn, these considerations force us to believe that 

 the primitive mouth of the ancestors of the vertebrates and of the 

 tunicates was situated in the fossa rhomboidea, where an cesophagus 

 pushed inwards to join the mid-gut, in the same way that it is 

 joined in insect embryos by the fore-gut. This primitive mouth 

 and its oesophagus were homologous with the corresponding organs 

 of modern arthropods and annelids. The mouth of the modern 

 vertebrates is then to be regarded as a secondary mouth, which has 

 gradually supplanted and replaced the old one on account of its 

 greater efficiency. 



It follows from this, according to Dohrn (p. 56), that the 

 " so-called larva " of the ascidians is a degenerated fish, and that 

 all the features which show the derivation of the cyclostomes from 



