126 EXPLAXATIOX OF THE PLATES. 



distinct from the digestive cavity than in any of the former stages. There is a slight constriction at 

 their base of attachment, the first indication of their final separation from the alimentary canal. 



Fig. 14. Isolated digestive cavity seen endwise, to show the tube leading from the mouth to the digestive 

 cavity, at right angles to the pouch of the earlets. 



Fig. 15. Isolated digestive cavity seen from above, in which the earlets (w, w') (the future water-tubes) 

 are so far differentiated as to be quite distinct from the digestive cavity. The walls of the earlets are 

 exceedingly attenuated, and are scarcely connected with the main digestive cavity. 



Fig. 16. The same as Fig. 15, seen from below, to show the position of the mouth and anus on tlie same 

 side of the larva. 



Fig. 1 7. Part of the same larva seen in profile : on account of tlie obliquity of tlie earlets, one of them 

 (w'), as it increases in size more rapidly than the other, soon reaches the outer surface of the lana 

 and opens into the surrounding medium by means of a small aperture (6). The walls of the tube 

 (fEsophagus) leading from the mouth to the first swelling of the digestive cavity (d) (the stomach), 

 and of that part of the tube leading from the stomach to the anus, have a very different thickness. 

 Thfy are sufficiently distinct in their character to enable us to distingubh readily three regions ; forty- 

 eight hours after fecundation. 



Fig. 18. The two small bodies (w, w'), the former earlets of younger stages formed from the pouch at the 

 closed end of the digestive cavity (the problematic bodies of Miiller), have entirely separated from 

 the digestive cavity from which they were formed ; seen from above, the three divisions of stomach, 

 intestine, and oesophagus are plainly marked out 



Fig. 1 9. The same larva in profile. 



Fig. 20. The same figure from below, shows the presence of short crescents of vibratile cilia (r, v') placed 

 in opposite directions near the mouth and anus; sixty-five hours after fecundation. 



Fig. 21. A somewhat more advanced larva, seen in profile; the anal crescent («) of vibratile cilia is seen 

 as a small wart between the mouth of the anu*, the oral crescent (t') projects beyond the general out- 

 line. The division into oesophagus (o), stomach {d), and intestine (c) is quite prominent. The 

 stomach has a tendency to approach the anal dorsal extremity. 



Fig. 22. llie same as Fig. 21, seen from below, to show the triangular shape of the mouth (>«)• Tlie 

 greater size of the problematic bodies (w, w') (the water-tubes), which increase independently at an 

 unecjual rate, and also the position of the oral and anal vil)ratile crescents. 



Fig. 23. The same larva seen in a profile, to show the position of the mouth in a strongly marked depres- 

 sion ; the great in<'rease in size of the oral part of the oesophagus ; the swelling out of the stomach, 

 and the bending of the intestine back towards the mouth, so as to make a small angle with the 

 tn-nd of the stomach ; at the end of (he third day after fecundation. 



Fig. 24. Larva seen from above. The only difference in this stage from the preceding is in the greater 

 increase of the vibratile crescents, forming two small plastrons, and of the water- tube. Tlic intes- 

 tine also bends so as to make almost a right angle with the stomach, which is pushed out further 

 towards the anal extremity. 



Fig. 25. More advanced larva, seen from the left profile, in which the oral pouch has assumed its charac- 

 teristic pistol-shape. The htomach and intestine make a sharp angle with each other, the latter being 

 nuich longer than the stomach proper. In its present aspect it closely resembles a retort, the stomach 

 being the receiver, the intestine the tube. The anal and oral vibratile crescents have greatly e.x- 

 tended, the one on the oral ami the other on the dorsal side, to the extremity of the body. 



Fig. 26. The same as the preceding, seen from below; the oral plastron is quite large, projects V>eyond 

 the sides of the body ; slight indentations can already be traced in the an.il plastron, indicating the 

 position of the future arms (e'). The water-tubes have increased in length, and extend halfway 

 from tlie base of the stomach to the oral plastron. 



Fig. 27, A larva six days after fecundation, seen from the right profile, the water-tubes extend beyond the 

 opening of the mouth. The tube leading from the water-pore (i) (dorsal iK)n>) to the water-tube 

 («•'), is (juite <listinctly seen. 



Fig. 28. The same larva as Fig. 27, seen from below ; the intestine, as in Fig. 26. is thrown to one side 

 of the axis of {he larva. The water-tubes extend also along the siiles of the stomach towards the anal 

 extremity ; the sinuosity of the anal ciliary chord indicates the rnisition of the future anus. 



