ORIGImX of the GEHMISAL layers. 28 1 



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 invaoination. It 

 must, however, be borne in mind that the frequency of the process of 

 invagination admits of another possible explanation. There is a con- 

 tinual 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 de- 

 lamination 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 re- 

 garding 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^: 

 Ccelenterata. — Pelagia, Cereantlms. 



Turhellaria. — Leptoplana (?), Thysanozoon. 



Nemertea. — Vilvlhxra, larvae of the type of Desor. 



MoUusca. — In numerous examples of most MoUuscan groups, except the 



Cephalopoda. 

 Chcetopoda. — Most Oligochasta, and probably many Polychaeta. 

 Gephyrea. — Phascolosoma, Phoronis. 

 Nematelminthes. — Cucullanus. 



(2) It closes in the position where the mouth is subsequently formed. 

 Ccelenterata. — Ctenophora (?). 



MoUusca. — In numerous examples of most Molluscan groups, except the 



Cephalopoda. 

 Crustacea. — Cirripedia (?), some Cladocera (Moina) (?). 

 (.3) It becomes the permanent anus. 

 MoUusca. — Paludina. 



Chtetopoda. — 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. — Pon'fera — Sycandra. 

 Ccelenterata — Chrysaora*, Aurelia*. Nemertea* — Some larvae which develop without 



1 The above list is somewhat tentative ; and future investigations will probably 

 shew that many of the statements at present current about the position of the blasto- 

 pore 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. 



