(4) 



EXPLANATION OF THE PLATES. 



last-formed opening), passes through the narrow coni- 

 cal tube (o) into the digestive cavity (d), communi- 

 cating with the earlets (w, to'), and out through the anal 

 opening (a), which was the first formed, and formerly 

 performed the functions of mouth. 



Fig. 11. Isolated digestive cavity, seen in profile, showing 

 the tube leading from the mouth (m) to the digestive 

 cavity (d), and earlets (to, to'), more developed than 

 in Fig. 10. 



Fig. 12. The same seen from above. 



Fig. 13. Oral end of an isolated digestive cavity, in 

 which the earlets, formed by the pouch, are more 

 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, to') (the future water-tubes) are 

 so far differentiated as to be quite distinct from the 

 digestive cavity- The walls of the earlets are ex- 

 ceedingly 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 the same side 

 of the larva. 



Fig. 17. Part of the same larva seen in profile ; on 

 account of the obliquity of the earlets, one of them 

 (to'), as it increases in size more rapidly than the 

 other, soon reaches the outer surface of the .larva 

 and opens into the surrounding medium by means 

 of a small aperture (b). The walls of the tube 

 (cesophagus) leading from the mouth to the first swell- 

 ing 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. They are 

 sufficiently distinct in their character to enable us to 

 distinguish readily three regions ; forty-eight hours after 

 fecundation. 



Fig. 18. The two small bodies (w, to'), 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 oesoph- 

 agus are plainly marked out. 



Fig. 19. The same larva in profile. 



Fig. 20. The same figure from below, shows the presence 



of short crescents of vibratile cilia (v, v>) placed in 

 opposite directions near the mouth and anus ; sixty- 

 five hours after fecundation. 



Fig. 21. A somewhat more advanced larva, seen in pro- 

 file ; the anal crescent (o) of vibratile cilia is seen as 

 a small wart between the mouth of the anus, the oral 

 crescent (o') projects beyond the general outline. The 

 division into oesophagus (o), stomach (d), and intes- 

 tine (c) is quite prominent. The stomach has a ten- 

 dency to approach the anal dorsal extremity. 



Fig. 22. The %ame as Fig. 21, seen from below, to show 

 the triangular shape of -the mouth (m). The greater 

 size of the problematic bodies (to, w>) (the water- 

 tubes), which increase independently at an unequal 

 rate, and also the position of the oral and anal vibra- 

 tile crescents. 



Fig. 23. The same larva seen in a profile, to show the 

 position of the mouth in a strongly-marked depression ; 

 the great increase in size of the oral part of the 

 cesophagus; 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 trend of the 

 stomach; at the end of the third day after fecunda- 

 tion. 



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. The intestine 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 pro- 

 file, in which the oral pouch has assumed its charac- 

 teristic pistol-shape. The stomach and intestine make 

 a sharp angle with each other, the latter being much 

 longer than the stomach proper. In its present as- 

 pect it closely resembles a retort, the stomach being 

 the receiver, the intestine the tube. The anal and 

 oral vibratile crescents have greatly extended, the 

 one on the oral and 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 beyond the 

 sides of the body; slight indentations can already be 

 traced in the anal plastron, indicating the position of 

 the future arms (e'). The water-tubes have in- 

 creased in length, and extend half way from the 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 



