The relative frequency of the two processes supplies us with another possible means for deciding between them; and there is no doubt that here again the scale inclines towards invagination. It must, however, be borne in mind that the frequency of the process of invagination admits of another possible explanation. There is a continual tendency for the processes of development to be abbreviated and simplified, and it is quite possible that the frequent occurrence of invagination is due to the fact of its being, in most cases, the simplest means by which the two-layered condition can be reached. But this argument can have but little weight until it can be shewn in each case that invagination is a simpler process than delamination; and it is rendered improbable by the cases already mentioned in which delamination has been secondarily derived from invagination. If it were the case that the blastopore had _in all types_ the same relation to the adult mouth, there would be strong grounds for regarding the invaginate gastrula as an ancestral form; but the fact that this is by no means so is an argument of great weight in favour of some other explanation of the frequency of invagination. The force of this consideration can best be displayed by a short summary of the fate of the blastopore in different forms. The fate of the blastopore is so variable that it is difficult even to classify the cases which have been described. (1) It becomes the permanent mouth in the following forms[120]: _Coelenterata._--Pelagia, Cereanthus. _Turbellaria._--Leptoplana (?), Thysanozoon. _Nemertea._--Pilidium, larvæ of the type of Desor. _Mollusca._--In numerous examples of most Molluscan groups, except the Cephalopoda. _Chætopoda._--Most Oligochæta, and probably many Polychæta. _Gephyrea._--Phascolosoma, Phoronis. _Nematelminthes._--Cucullanus. (2) It closes in the position where the mouth is subsequently formed. _Coelenterata._--Ctenophora (?). _Mollusca._--In numerous examples of most Molluscan groups, except the Cephalopoda. _Crustacea._--Cirripedia (?), some Cladocera (Moina) (?). (3) It becomes the permanent anus. _Mollusca._--Paludina. _Chætopoda._--Serpula and some other types. _Echinodermata._--Almost universally, except amongst the Crinoidea. (4) It closes in the position where the anus is subsequently formed. _Echinodermata._--Crinoidea. (5) It closes in a position which does not correspond or is not known to correspond[121] either with the future mouth or anus.--_Porifera_--Sycandra. _Coelenterata_--Chrysaora*, Aurelia*. _Nemertea*_--Some larvæ which develop without a metamorphosis. _Rotifera*. Mollusca_--Cephalopoda. _Polyzoa*._ _Brachiopoda_--Argiope, Terebratula, Terebratulina. _Chætopoda_--Euaxes. _Discophora_--Clepsine. _Gephyrea_--Bonellia*. _Chætognatha. Crustacea_--Decapoda. _Chordata._