414 TH E EVOLUTION OF MAX. 



Both stand on the borderland between the two chief divisions 

 of the animal kingdom, which since the time of Lamarck 

 (1801) have been distinguished as the Vertebrates and 

 the Invertebrates. The Vertebrates embrace the already 

 mentioned classes from the Lancelet up to Man (Acephala, 

 Lampreys, Fishes, Double-breathers, or Dipneusta, Amphibia, 

 Reptiles, Birds, Mammals). In contradistinction to these, all 

 other animals have usually, in agreement with the example 

 of Lamarck, been classed as " Invertebrates." But, as we 

 have already had occasion to remark, the Invertebrates in 

 turn consist of several quite distinct tribes. Of these, the 

 Star-animals (Ecliinodervia), the Soft-bodied Animals (Mol- 

 lusca), and the Articulated Animals (Arthropoda), do not 

 interest us here, because they are independent main branches 

 of the animal genealogical tree, which are quite distinct 

 from the Vertebrates. The class of Worms is, on the 

 other hand, extremely interesting to us. In this group 

 a very remarkable class of animals exists which has only 

 recently been carefully studied, and which bears most 

 significantly on the genealogical tree of Vertebrates. This 

 class is that of the Mantle-animals (Tunicata). One 

 member of this class, the Sea-squirts (Ascidia), very closely 

 resembles in its internal structure and in its germination 

 the lowest Vertebrate, the Lancelet (Amphioxus). Till a 

 few years ago no one suspected the close connection be- 

 tween these two apparently quite different animal forms, 

 and it was a very lucky accident that just now, while the 

 question as to the descent of the Vertebrates from Inverte- 

 brates is foremost, the germ-history of these two most 

 closely allied animals was discovered. In order rightly to 

 understand the germ-history of the Lancelet and the Sea- 



