It is very difficult to bring forward arguments of a conclusive kind in favour of either of these processes. The fact that delaminate and invaginate gastrulæ are in several instances found coexisting in the same group renders it certain that there are not two independent phyla of the Metazoa, derived respectively from an invaginate and a delaminate gastrula[119].
The four most important cases in which the two processes coexist are the Porifera, the Cœlenterata, the Nemertea, and the Brachiopoda. In the cases of the Porifera and Cœlenterata, there do not appear to me to be any means of deciding which of these processes is derived from the other; but in the Nemertea and the Brachiopoda the case is different. In all the types of Nemertea in which the development is relatively not abbreviated there is an invaginate gastrula, while in the types with a greatly abbreviated development there is a delaminate gastrula. It would seem to follow from this that a delaminate gastrula has here been a secondary result of an abbreviation in the development. In the Brachiopoda, again, the majority of types develop by a process of invagination, while Thecidium appears to develop by delamination; here also the delaminate type would appear to be secondarily derived from the invaginate.
If these considerations are justified, delamination must be in some instances secondarily derived from invagination; and this fact is so far an argument in favour of the more primitive nature of invagination; though it by no means follows that in the invaginate process the steps by which the Metazoa were derived from the Protozoa are preserved.
It does not, therefore, seem possible to decide conclusively in favour of either of these processes by a comparison of the cases where they occur in the same groups.
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]:
Cœlenterata.—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.
Cœlenterata.—Ctenophora (?).
Mollusca.—In numerous examples of most Molluscan groups, except the Cephalopoda.
Crustacea.—Cirripedia (?), some Cladocera (Moina) (?).