PLEUROBRACHIA RHODODACTYLA. 



31 



ties of any kind. The position of the ambulacral system is eai'ly well 

 defined by four short double rows of combs, each row not having more 

 than three or four combs (Figs. 38-41). The spherosome early shows 

 the great difference in the size of the longitudinal and coeliac diameters, 

 the tentacular diameter being nearly twice as long as the other (Figs. 

 40, 42). The locomotive rows extend rapidly to the level of the upper 

 part of the digestive cavity. At this time the ambulacral cavity makes 

 its appearance as a small spherical cavity, in the same way as the diges- 

 tive cavity. The ambulacral cavity increases rap- 

 idly, soon attains the size of the digestive cavity, 

 and occupies the whole of the abactinal extrem- 

 ity of the animal (Fig. 43, o). At this time the 

 young Pleurobrachia is quite pear-shaped, with 

 sohd tentacles about as long as the polar diam- 

 eter. The ocular sphere is lai-ge, very promi- 

 nent (Fig. 43, e). There are two large elliptical 

 cavities, of nearly equal size (Fig. 43, o, d). In 

 the next stage the two cavities differ in their 

 outline, the ambulacral cavity becoming more 

 and more rectangular, and the digestive cavity 

 triangular, the two being separated by a wall which grows thinner and 

 thinner. The combs of the ambulacral rows increase in size, and the 

 flappers are quite long, equallmg in length half the transverse diameter 

 of the spherosome. The ambulacral cavity extends towards the abac- 

 tinal region on both sides of the thickening of the wall, supporting the 

 sensitive bulb. This is the first sign of the for- 

 mation of the funnel (Fig. 44,/), and its division 

 into the two branches, opening outwards. The 

 compression of the digestive cavity is plainly 

 seen at this stage, as when seen facing the ten- 

 tacles the cavity comes close to the outer wall 

 (Fig. 44), while when seen at right angles to 

 the tentacular diameter it occupies but a much 

 smaller space (Fig. 45). The tentacular bulb becomes more isolated, 

 the tentacle is about three tunes as long as the polar diameter. During 

 all this time, and from the first appearance of the locomotive flappers, 

 the young Medusa moves about with the greatest rapidity, turning over 

 in every possible direction, running round and round, with the diges- 

 tive cavity forward, in the envelope of the egg, as if trying to make 

 its escape from it ; while at other times the young Medusa remains 



Fig. 44. Pleurobrachia in wliicli the digestive cavity and the ambulacral cavity are already 

 connected, immediately before the escape of the Medusa from the egg. 



Fig. 45. Same as Fig. 44, somewhat less magnified, to show the relative size of egg envelope 

 and embrj'o. 



