THE INVERTEBRATE PHYLA 103 



In the Mollusca the fate of the blastopore also varies. In the 

 Gastropoda as a rule the anterior part of the blastopore gives rise 

 to the mouth, in Paludina the mouth arises from the posterior 

 part of the blastopore. In all gastropods the anus is a new 

 formation unrelated to the blastopore. In the Amphineuran 

 Ischnochiton, too, the anus is a new formation. The lamellibranches 

 such as Teredo or Cyclas have the blastopore closed completely 

 and the mouth and anus are entirely new formations. Otherwise 

 the blastopore becomes the site of the mouth. Further informa- 

 tion can be found in DawydorT (1928) and Manton (1948). 



In the Deuterostomia the fate of the blastopore is similarly 

 varied. The echinoderm mouth is a new formation in the larva 

 whilst the blastopore becomes the larval anus. The same is true 

 for the hemichordates. In the Tunicata and the Cephalochordata 

 the blastopore becomes dorsally placed to form the neuropore. In 

 the Chaetognatha the blastopore closes and does not become 

 either the mouth or the anus. 



It can be seen that the division into Protostomia and Deuteros- 

 tomia is not as sharp as might be expected. Thus certain groups 

 such as the Tardigrada, Chaetognatha, Tunicata, Cephalochordata 

 and so on would be in neither the Protostomia nor the Deuteros- 

 tomia. In other groups such as the Annelida or Arthropoda, 

 certain genera have the blastopore forming the mouth whilst 

 others do not. 



This situation was appreciated quite early. Thus Sedgwick 

 stated in 1915, " In Peripatus the mouth and anus are not only 

 derived from the elongated blastopore by its constriction into two 

 openings but remain throughout life included within the nerve 

 ring derived from the neural rudiments of the embryo. If in 

 other Arthropoda, in Annelida, and in the Mollusca we find, as 

 we do, that the nerve ring referred to is, in the adult, incomplete 

 behind the anus, and the mouth and anus, though obviously 

 referable to the blastopore, are not actually 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} To maintain such a position 

 appears to us impossible and we entirely accept the doctrine that 

 the mouth and anus of the Annelida, Arthropoda, and Mollusca 

 are both perforations of the embryonic neural surface and are 



