340 BLASTOPORE. 



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 1 : 

 Ccelenterata. Pelagia, Cereanthus. 



Turbellaria. Leptoplana (?), Thysanozoon. 



Nemertea. Pilidium, larvae of the type of Desor. 



Mollusca. In numerous examples of most Molluscan groups, except the 



Cephalopoda. 



Chatopoda. Most Oligochaeta, and probably many Polychseta. 

 Gephyrea. Phascolosoma, Phoronis. 

 Nematelminthes. Cucullanus. 



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

 Ccelenterata. 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. 



Ch&topoda. 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 2 either with the future mouth or anus. Porifera Sycandra. 

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

 a metamorphosis. Rotifera* '. Mollusca Cephalopoda. Polyzoa*. Brachiopoda 

 Argiope, Terebratula, Terebratulina. Chatopoda Euaxes. Discophora Clepsine. 

 Gephyrea Bonellia*. Chcetognatha. Crustacea Decapoda. Chordata. 



The forms which have been classed together under the last 

 heading vary considerably in the character of the blastopore. 

 In some cases the fact of its not coinciding either with the mouth 



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. 



2 The forms in which the position of the blastopore in relation to the mouth or 

 anus is not known are marked with an asterisk. 



