ON THE DIPLOCHORDA. 401 



blastomereSj or, in other words, subequal segmentation. 

 Gastrulation is in each case total, and Bateson's fig, 5 

 would serve equally well for a stage of Phoronis, The 

 subsequent behaviour and fate of the blastopore is markedly 

 different. In Balanoglossus it closes up gradually and 

 completely, the hypoblast becoming completely separated 

 from the epiblast. Subsequently the anus opens at about 

 the same spot, as far as can be judged, as that at which the 

 blastopore closed, and the mouth appears ventrally as a new 

 structure. In the case of Tornaria the young stages do 

 not appear to have been followed prior to Goette's larva. In 

 Phoronis it is certain that the mouth is a persistent an- 

 terior portion of the blastopore, and it is pretty clear that 

 the anus opens subsequently at the posterior closed portion 

 of the blastopore. 



Schultze (16) emphasises the different fate of the blasto- 

 pore in criticising my comparison of Balanoglossus and 

 Phoronis. Those who accept the view that the blastopore 

 becomes phyletically both the mouth and anus of the 

 Ccelomata will not regard the fact that it persists in a 

 special ontogeny as mouth or anus, or both, or neither, as 

 having any bearing upon the question ; but for those who 

 hold other views one may recall the behaviour of the blasto- 

 pore in other groups. In the Echinodermata the blasto- 

 pore usually becomes the anus, but does not survive as mouth 

 or anus in Ante don, nor in Asterina. Again, within the 

 group of Gastropod Mollusca the blastopore becomes the 

 mouth in Patella (Patten), Chiton (Kowalevskij, and 

 Nassa (Bobretsky), whilst it survives as the anus in Palu- 

 dina (Lankester) and numerous others. 



In the face of such examples as this we can hardly attach 

 phyletic relationship or otherwise to a comparison of the 

 behaviour of the blastopore. In Chiton the blastopore 

 would appear to migrate from a terminal position, where the 

 anus opens later, to the position where it survives as the 

 mouth. In this, as in Balanoglossus Kowalevskii, it is 

 possible to follow this migration owing to the fact that there 



