178 SUMMARY. 



The exact period of leaving the egg does not appear to be very constant, 

 but the hatching never takes place till the embryo has practically acquired 

 all the organs of the adult. 



In the majority of types the differences between the just hatched larva 

 and the adult are inconsiderable, and in all cases the larva has a somewhat 

 oval form. In the case of the Taeniatae (Cestum, etc.), the larva has the 

 characteristic oval form, and the subsequent changes amount almost to a 

 metamorphosis. 



The larva of the Lobatas, such as Eucharis, Bolina, etc., can hardly be 

 distinguished from Pleurobrachia, and undergoes therefore considerable 

 changes after hatching. 



Eucharis multicornis while still in the larval condition is stated by Chun 

 to become sexually mature. 



The new genus Ctenaria recently described by Haeckel, 

 which is intermediate between the Ctenophora and the Medusae 

 clearly proves that the Ctenophora are more closely related to 

 the Medusae than to the Actinozoa ; but their development, 

 especially the presence of a stomodaeum, shews that they have 

 affinities (in spite of the rudimentary velum of Ctenaria) with the 

 Acraspedote as well as with the Craspedote Medusae ; and it 

 may be noted that the Acraspeda have undoubted affinities with 

 the Actinozoa. 



Summary and general considerations. 



Even in the adult condition the lower forms of Ccelenterata 

 do not rise in complexity much beyond a typical gastrula. 

 Ontogeny nevertheless brings clearly to light the existence of a 

 larval form the planula which recurs with fair constancy 

 amongst all the groups except the Ctenophora. 



We are probably justified in assuming that the planula is a 

 repetition of a free ancestral form of the Ccelenterata. The pla- 

 nula, as it most frequently occurs, is a two-layered ciliated nearly 

 cylindrical organism, with at most a rudimentary digestive cavity 

 hollowed out in the inner layer, and as a rule no mouth. In the 

 outer layer are numerous thread-cells. 



How many of these characters did the ancestral planula possess ? I think 

 it is not unreasonable to assume that the only two characters about which 

 there can be much doubt are the rudimentary condition of the digestive 

 cavity and the absence of a mouth. Paradoxical as it may seem, it appears to 

 me not impossible that the Ccelenterata may have had an ancestor in which a 

 digestive tract was physiologically replaced by a solid mass of amreboid cells. 



