4 CHORDATA. 



Arthropoda, in Annelida and in Mollusca we find, as we do, that 

 the nerve ring referred to is, in the adult, incomplete behind the 

 anus, and that the mouth and anus, though obviously referable 

 to the blastopore, are not actually both derived from it, must we 

 on this account deny this most obvious relation and maintain 

 that the mouth or anus, as the case may be, in these forms is 

 not homologous with that of Peripatus 'I 



To maintain such a position appears to us impossible, and we 

 entirely accept the doctrine that the mouth and anns of 

 the Annelida, Arthropoda and Mollusca are both perfor- 

 ations of the embryonic neural surface and are specialisations 

 of parts of one original opening which is represented in most 

 embryos by the blastopore. When however we come to apply 

 this doctrine to the Chordata we stand upon more debatable 

 ground. Placing the Enteropneusta on one side as not ob- 

 viously conforming to our plan, we find that it is a fact of 

 observation that in the Chordata the blastopore perforates the 

 embryonic nerve rudiment, and that in some of them the anus 

 is directly derived from it (many Pisces, some Amphibia, 

 e.g. newt), whereas in others, not at all remote from these, 

 the blastopore closes entirely and the anus is a new formation 

 (some Pisces and Amphibia, e.g. frog, Amniota). Here also 

 we think it may fairly be maintained that notwithstanding 

 the diversity in the mode of develoj)ment of the anus it is, in all 

 Vertebrata at least, a derivate of the blastopore. The non- 

 inclusion of the anus within the nerve rudiment in the adult, 

 and its shift on to the ventral smiace, carmot be brought against 

 this view, because these facts apply both to animals in which 

 the anus is a persistent part of the blastopore, as well as to 

 those in which it is a new formation. Here again, as in the 

 invertebrate phyla already dealt with in this connection, the 

 anus escapes in the adult from the embryonic nerve rudiment ; 

 or to put it in another way the part of the nerve rudiment behind 

 the anus never attains full development, but early undergoes 

 atrophy.* So far then all is plain sailing, in the Vertebrata at 

 least : the anus is a persistent portion — not the whole, as is clear 

 from a consideration of the development of Elasmobranchs and 

 some Amphibia — of the blastopore, as it is in the invertebrate 



* See especially Lepidosiren, in which the medullary folds of the embryo 

 include the blastopore which becomes the anus. 



