Terminal layers of the embryo. 256 
the processes of development to be abbreviated and simpli- 
fied, 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 shown that invagination is a simpler process 
than delamination. 
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 argu- 
ment 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 diffi- 
cult even to classify the cases which have been described. 
The forms which have been classed together under the 
(1) It becomes the permanent mouth in the following forms d 
Ccelenterata. — Pelagia, Cereanthus. 
Turbellaria. — Leptoplana (?), Thysanozoou. 
Nemertea. — ^Pilidium, larvae of the type of Desor. 
MoUusca. — In numerous examples of most molluscan groups, ex- 
cept the Cephalopoda. 
Chcetopoda. — Most Oligochseta, and probably many Polychseta. 
Gephyrea. — Phascolosoma, Phoronis. 
Nematelminthes. — Cucullanus. 
(2) It closes in the position where the mouth is subsequently formed. 
Ccelenterata.—Citm\}h.ov^ (?). 
MoUusca. — In numerous examples of most Molluscan groups, except 
the Cephalopoda. 
Crustacea. — Cirripedia (?), some Cladocera (Moiiia) (?). 
(3) It becomes the permanent anus. 
MoUusca. — Paludina. 
Chcetopoda. — 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^ either with the future mouth or anus. — Porifera : Sycandra. 
Ccctenierata : Chrysaora,* Aurelia.* Nemertea ;* some larvae which develop 
without a metamorphosis, liotifera.* MoUusca : Cephalopoda. Polyzoa.* 
Brachiopoda: Argiope, Terebratula, Terebratulina. Chcetopoda: Euaxes. 
Discophora : Clepsine. Gephyrea : Bonellia.* Chcetognatha. Crustacea : 
Decapoda. Chordata. 
‘ The above list is somewhat tentative ; and future investigations will 
probably show that many of the statements at present current about the 
position ol the blastopore are inaccurate. 
^ The forms in which the position of the blastopore in relation to the 
mouth or anus is not known are marked with an asterisk. 
