DEVELOPMENT OP HALISAECA LOBULARIS. 613 



either by invagination or delamination, is a remarkable point 

 of resemblance between the Sponges and the Metazoa. 



Indeed, on summing up the resemblances between Sponges 

 and Metazoa one inclines to remain in doubt whether it is 

 more probable that the metazoan or the infusorian characters 

 should have been independently evolved. On the whole, I 

 incline myself to regard the metazoan characters as '' homo- 

 plastic/' to use Lankester's term, and the infusorian as phy- 

 logenetic. The two chief metazoan characters are the presence 

 of both sexual elements and the formation of a gastrula. With 

 regard to the former we know next to nothing as to the 

 causes which have led to sexual differentiation, but we have 

 every reason to suppose that it has originated independently in 

 animals and plants, and if so it might just as well have done so 

 in the case of Sponges and Coelenterates. The formation of 

 the gastrula is a different matter, and here it is to be remarked, 

 first, that the choano-flagellate cells of the Sponge make their 

 appearance very early in development, before the formation 

 of a gastrula, as is clearly the case in the amphi-blastula of 

 Sycandra raphanus; secondly, folding is one of the com- 

 monest phenomena in various stages in the development of all 

 animals, and is probably susceptible of a very simple mechanical 

 explanation ; at all events in a large number of cases similar 

 foldings originate quite independently of heredity, and give rise 

 to homoplastic organs which are certainly not homologous. 

 It is scarcely necessary to cite cases, they are so numerous ; but 

 one may instance the formation by folding of the eye in 

 Molluscs, Worms, Peripatus, and Vetebrates as a case in point. 

 Furthermore, both in Sponges and Coelenterates the formation 

 of the gastrula takes place in two ways, by invagination, as in 

 Sycandra, and by fission of the mesenchyme, as in Plakina 

 among Sponges and Eucope among Coelenterates. Its origin 

 in at least one of these ways is inexplicable directly from here- 

 dity. For if for the sake of argument we assume both Sponges 

 and Coelenterata to have had a common ancestor, a gastrula 

 which originated by invagination, we must then suppose that 

 the formation of a gastrula by fission in each group arose as an 



VOL. XXIV. NEW SEB, T T 



