IV 



COELENTEKATA 



95 



independent evaginations. All grow down towards the edge of the 

 mouth and here each one forks, and adjacent forks unite with one 

 another, and so a circular canal is formed. This is a peculiarity 

 of Berue as opposed to other Ctenophora. From the gastric sac 

 two canals extend upwards towards the aboral pole of the larva and 

 fuse with the ectoderm at the sides of the apical plate. Here open- 

 ings are made to the exterior. These canals are called excretory 

 canals, and out of them a current of water flows (Fig. 77). It is 

 probable that this current forms the exhalent portion ofa respiratory 

 current, and that water enters 

 by the mouth to replace it. 

 Finally, the meridional canals 

 give off short outgrowths 

 which, in one species, anasto- 

 mose with one another so as 

 to form a network. 



CALLIAN1KA 



The main difference 

 between the development of 

 Beroe and that of other Cteno- 

 phora lies in the presence of 

 tentacles more or less modified 

 in the other groups. 



The best account of the 

 development is that given by 

 Metschnikoff of Callianira 

 (1885). From this account 

 we learn that the two main 

 arms of the " mesodermal 

 cross," after having given off 

 a certain number of wander- 

 ing cells, give rise to the tissue 

 which forms the axis of the 

 tentacles. The tentacles themselves appear as invaginations the 

 tentacle pockets from the base of which the real tentacle sprouts. 

 These tentacle pockets indent the endodermic sac in a plane at right 

 angles to that in which it is indented by the stomodaeum. 



m 



FIG. 78. The free-swimming larva of (Jllitniii-n 

 liinlata viewed from the stomach-plane. (After 

 Metschnikoff. ) 



tun, freely projecting tentacle ; ///, wandering cells 

 with contractile processes. 



AFFINITIES OF CTENOPHORA WITH OTHER COELENTEliATA 



Now, when we review the account which has just been given of 

 the development of Beroe, there is really only one point where any 

 marked similarity with the developmental stages of other Coelenterata 

 shows itself, and this is when the ectoderm has completely invested 

 the macromeres, and these latter have broken up into a mass of cells 

 like plant " parenchyma," with only slits between them. This staye 



