THE "ROUND-MOUTHS." 427 



whole century, even beyond Pallas (1778), and only proves 

 that his notions of Comparative Anatomy and of the history 

 of evolution are extremely weak. 



The Amphioxus does, indeed, stand very far below all 

 other extant Vertebrates. It is, indeed, without the head 

 containing a developed brain and skull, which distinguishes 

 all other Vertebrates. It is without an organ of hearing, 

 and without a centralized heart, such as all others possess ; 

 perfect kidneys are also lacking. Each organ appears in a 

 simpler and more imperfect form than in any other Ver- 

 tebrate. And yet, the rudimentary characters, the connec- 

 tion and relative position of all the organs, are the same as 

 in all other Vertebrates : moreover, they all, during their 

 embryonic development, pass, at an early period, through a 

 stage in which their whole organization is not superior to 

 that of the Amphioxus, but rather, agrees with it in all 

 essential particulars. (Cf. Table IX.) 



In order to be thoroughly convinced of this important 

 fact, it is specially instructive to compare the Amphioxus 

 with the early forms of development of those Vertebrates 

 which are most nearly allied to it in the natural system 

 of this tribe. This is the class of the Round-Mouths 

 (Cyclostomi). This remarkable class, which formerly com- 

 prehended many species, contains at the present day but 

 very few species, which are separable into two different 

 groups. One group is formed by the Hags (Myxinoidce), 

 which have been made known to us by Johannes Muller's 

 classic work, " Vergleichende Anatomie der Myxinoiden." 

 The other group is formed by the well-known Lampreys, or 

 Rock-Suckers (Petromyzonta), which are eaten as a delicacy. 



All these Round-Mouths are usually included in the class of 

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