CCELENTERATA. 147 



(Chun), of a contractile nature. It is probable that the great mass of 

 the gelatinous tissue of the adult is an intercellular substance derived 

 from these cells. 



The whole of the above changes are completed while the embryo 

 is still enclosed in the egg capsule. During their accomplishment 

 the oro-anal axis, which was originally very short, increases greatly 

 in length (fig. 83), so that the embryo acquires an oval form similar 

 to that of the adult. 



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

 but the hatching never takes place till the etnbryo 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 Tseniatse (Cestum, etc.), the larva has the 

 characteristic oval form, and the subsequent changes amount almost to 

 a metamorphosis. 



The larva of the Lobatse, 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 Medusas clearly 

 proves that the Ctenophora are more closely related to the Medusas 

 than to the Actinozoa ; but their development, especially the presence 

 of a stomodseum, shows that they have affinities (in spite of the rudi- 

 mentary velum of Ctenaria) with the Acraspedote as well as with 

 the Craspedote Medusas; 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 Cteuopbora. 



We are probably justified in assuming that the planula is a 

 repetition of a free ancestral form of the Ccelenterata. The planula, 

 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 1 

 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 



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