94 INVEETEBEATA CHAP. 



if not most, of the cell -processes are contractile, and therefore these 

 wandering cells are to be regarded as muscular. 



In the endoderm there appear slit-like spaces between the cells 

 here and there, and these now coalesce so as to form a single roomy, 

 gastric cavity. At the lower pole the stomodaeum now makes its 

 appearance as an invagination of the ectoderm. This, as it grows 

 upwards, indents the endodermic sac and constricts it into right and 

 left halves. An indentation occurs in this sac at each side of a line 

 which is at right angles to the plane of the stomodaeuui, and in this 

 way a four-lobed sac is formed. We might with perfect justice 

 describe the process as a formation of four ridges or taeniolae, which 

 grow into and indent the cavity of the endoderm sac. 



Meanwhile, the " combs " of cilia have appeared in the regions, 

 where, at an earlier stage, the streaks of rapidly-dividing ectoderm 



ex 

 s.st 



FIG. 77. Part of apical region of larva of Beroe forskalii viewed from stomach-plane. 



(After Chun.) 



Letters as before, ex, excretory vessel rising from the funnel. 



cells were noticeable. Thus the ribs of the adult are established. At 

 the upper pole, groups of stiff cilia supporting otolithic masses are 

 formed, and in this way the apical sense-organ is completed. 



We have now an oval embryo containing a four-lobed gastric sac, 

 whose walls are formed of vacuolated cells with stiff membranes, and 

 having on one side a mouth leading into a flattened stomodaeum 

 which projects inwards into its interior. Eunning like the meridians 

 on a globe are eight ribs of thickened ectoderm, each bearing a series 

 of combs of cilia, and converging to a polar plate at the pole opposite 

 the mouth : this plate carries groups of stiff converging cilia which 

 support an otolith where they meet. 



At this stage the embryo escapes from the egg-membrane and 

 begins life as a free-swimming larva. The later history has been 

 followed only by Chun and has been deduced from the examination of 

 specimens caught by the Plankton net. From these he concludes 

 that the meridional canals appear on the periphery of the endodermic 

 pouches as everted grooves. First the sub-tentacular and then the 

 sub-stomachic canals appear, then the two paragastric canals arise as 



