32 



PLEUROBRACHIA RHODODACTYLA. 



Fig. 46. 



Fig. 47. 



poised in the centre of the egg, rotating slowly on its axis, imitating, 

 while still in the egg, all the movements which are so characteristic of 

 the adult. The young Medusa, before it escapes from the egg, occupies 

 a comparatively small space, having thus ample room for its manifold 

 movements. In Fig. 44 the outline of a part of the egg is seen ; Fig. 45 

 is the same as Fig. 44 from the opposite side, showing the whole egg. 



Before the young leaves the egg, we 

 find that the ambulacral cavity and 

 digestive cavity connect by means of 

 a small opening in the centre of the 

 dividing wall, and at the same time a 

 depression at the actinal pole soon 

 increases sufficiently to pierce through 

 the wall, and make an opening, 

 the mouth (Fig. 46). The young 

 Pleurobrachia now makes its escape 

 from the egg, and the changes it undergoes are very rapid ; the funnel 

 becomes well isolated, and the digestive cavity quite compressed, and 

 we see the first sign of the separation of the double row of locomotive 

 flappers into two very distinct rows. At the same time, when facing 



the tentacular bulb, we see a small triangular 

 pouch extending along the digestive cavity, 

 which, when seen in profile, plainly appears 

 to be nothing but a coecum of the ambulacral 

 cavity, formed exactly as in Bolina (Fig. 5). 

 These pouches are the rudimentary lateral 

 chymiferous tubes so characteristic of Cteno- 

 phorae. At this stage the ambulacral flappers 

 are not as near the abactinal pole as in 

 former stages, on account of the elongation 

 of portions of the spherosome. The lateral 

 tubes increase rapidly in length, and soon 

 extend to the level of the mouth (Fig. 47, ), 

 while the forking of the ambulacral tubes becomes more deep. We 

 notice also at this time a marked difference in the size of the ambu- 

 lacral tubes. The tentacular ambulacra (those on each side of the 

 tentacular apparatus) are much shorter than the longitudinal ambulacra 

 (Fig. 48, c). The tentacle, also, is no longer a simple solid thread ; 

 long, slender offshoots, similar to the tentacle, have developed near the 



Fig. 46. Same as Fig. 44, seen from actinal side. 



In all the preceding figures the embryo has been drawn without the egg envelope ; but it must 

 be remembered that the little Medusa does not escape from the egg till it reaches the condition of 

 Fig. 44. 



Fig. 47. Pleurobrachia swimming freely about, in which the lateral tubes, the funnel, have 

 become highly developed ; seen from the broad side. 



