12 THE CTENOPHORA 



invagination by concrescence of the epiblast at the upper pole, 

 and the embryo is now a gastrula (10). A secondary invagination 

 of the ectoderm gives rise to a stomodaeum, and the mesoblast 

 cells travel to the aboral pole and spread out beneath the ectoderm 

 to form a plate of cells from which all the muscles of the body are 

 eventually developed. The tentacles are first seen as thickenings 

 of the ectoderm in the transverse plane, to which two plates of 

 mesoblast attach themselves. The mesoblast plates extend not 

 only in the transverse, but also in the sagittal plane, so that a 

 cross-shaped figure is formed, the exact significance of which is 

 not known (12). It is supposed by some that it is an indication 

 of the existence of sagittal tentacles in the ancestral Ctenophore. 

 The sense body is formed from an epiblastic thickening at the 

 aboral pole. The further stages of development can be understood 

 by reference to Fig. IV. 13, 14, and the reader is referred to 

 Metschnikoff's and Chun's works for details. 



All the Ctenophora reproduce themselves sexually. There is 

 no alternation of generations. In the Cydippidae and Beroidae 

 development is direct, but in the Lobatae and Cestidae there is a 

 metamorphosis. The larvae of these forms are cydippiform and 

 only gradually acquire their adult characters. In connection with 

 this metamorphosis a peculiar sequence of juvenile fertility, 

 adolescent sterility, and adult fertility has been observed in the 

 Lobatae, and has been named by Chun, its discoverer, Dissogony. 

 In the warm months the cydippiform larvae of Eucharis multicornis 

 and Bolina hydatina, as soon as they have escaped from the egg 

 membranes, and whilst they are only some '5 - *2 mm. in diameter, 

 become sexually mature and develop ova and spermatozoa in the 

 four sub-sagittal meridional canals. The ova are fertilised and give 

 rise to fresh cydippiform larvae. In the parent larva, after a brief 

 period of sexual activity, the gonads degenerate and a barren period 

 succeeds, during which the larva goes through a complicated meta- 

 morphosis. At the end of the metamorphosis the animal, now 

 much larger and indued with the full characters of a lobate Cteno- 

 phore, becomes a second time sexually mature, gonads being 

 developed in all the eight meridional canals (see Chun, 8). 



With few exceptions zoologists, since the time of Eschscholtz, 

 have been agreed in ranking the Ctenophora as a class of the 

 Coelentera, although much evidence has been brought forward 

 of late years to show that they have decided affinities with 

 Platyhelminthes, particularly with the Polyclada (see Lang, 17). 

 The polyclad affinities of Ctenophora are regarded as tending to 

 prove that the Polyclada are descended by way of the Cteno- 

 phora, or, at least, by way of a Ctenophore-like ancestor, from the 

 Coelentera. Such an argument implies +,hat the Ctenophora are 

 indubitably Coelentera. 



