342 E. W. MACBRIDE. 



tion. No one can suppose that the ancestral form gave up 

 its old mouth and developed a new one; the change was one of 

 size and relative position only. We must assume that the 

 original mouth was a wide one, and that part is utilised by the 

 larva and aborted in the adult. 



The coelom, as we have seen, arose as a specialised portion 

 of the gut. 



It is to be observed that the history we have just sketched 

 is in accordance with that rule which seems to hold in all cases 

 where we can by means of comparative anatomy show with 

 reasonable probability that evolution has occurred, viz. that 

 new organs never arise de novo, but by the differentiation of 

 older organs. This rule seems, however, to me to be violated 

 by supposing that either archenteron or coelom arose as a 

 split in a solid mass of cells. The history affords also an 

 explanation of that rigorous separation of primary and secondary 

 body cavities, the blastocoele or haemocoele, and the coelom, 

 which all recent research has tended to emphasise. The first 

 is, in fact, morphologically inside and the second outside the 

 primitive blastosphere. 



Lastly, the conception of the primitive metazoon as a colony 

 of Protozoa is in accordance with that repetition of similar 

 parts on which Bateson^ has laid so much stress as one of the 

 most marked characteristics of living things. We should 

 recall also the high individuality acquired by colonies of 

 Siphonophora, Polyzoa, and Ascidians. 



* ' Materials for the Study of Variation,' W. Bateson, Cambridge, 1890. 



