EXPLANATION OF TliK PLATES. 125 



Fig. 27. Twenty hours after fcctindation ; the depression has the appearance of a small poiuh ('I) hanjr- 

 in<^ in a pear-shaped body with cironlar se(!tion, sliowin-^ no deviation from flu; ahsolut*; radiate typ«i; 

 the openin'4 (a) still performing the double funetion-i of mouth and anm. Currents of water cin;ulate 

 in this cavity, as they would in the digestive cavity of any Polyp or Acaleph in about the same Biage 

 of development. 



Fig. 28. Twenty-two hours after fecundation ; the embryo has become somewhat more cylindrical, losing 

 its pear-shaped form, but is still circular when seen i.i a transverse section. Tlie cavity (tl) has 

 slightly expanded at th(i closed extremity, and is comparatively deeper and wider; the walls of the 

 body are much reduced in thickness, except at the perforated region. The body is somewhu translu- 

 cent, and slightly tinged with ochre color. Tlie opening (a) still serves as a mouth, although, in 

 more advanced stages, a second opening is formed, which is the true mouth, at which time the present 

 mouth then becomes the anus. 



PLATE II. 



In Figs 1, 3, 9-17, the digestive cavity alone is represented. 



Fig. 1. The digestive cavity of Fig. 2, seen by itself from above, has expanded into a large reservoir at 

 the extremity, the walls of which are quite thin. 



Fig. 2. The embryo of Fig. I seen in profile ; the cavity is no longer in the axis, but is bent to one side. 

 The larva has also lost its symmetrical outline, and the dorsal part of the perforated extremity projects 

 somewhat beyond the opening of the present mouth (the future anus). 



Fig. 3. The digestive sac of a larva somewhat more advanced than Fig. 2, in which the present mouth (a) 

 (the future anus) has been brought to the lower side. 



Fig. 4. The larva of Fig. 3 seen in profile : the pouch at the closed extremity of the bent digestive cavity 

 is now nearer the lower side than in Fig. 2, having approached the slight depression (m) placed in the 

 middle of the larva. 



Fig. 5. A larva somewhat more advanced, seen in profile, in which the pouch has actually come in con- 

 tact with the wall of the lower side at m^ The dorsal region of the perforated extremity projects 

 still more beyond the opening of the present mouth (a) (the future anus) than in tlie preceding 

 stage (Fig. 4). The digestive cavity is not yet divided into distinct regions. 



Fig. 6. The same larva as Fig. 5, seen from above, forty-two hours after fecundation ; large epithelia- 

 cells have appeared on the surface. 



Fig. 7. A somewhat more advanced larva, seen in profile ; the digestive cavity is no longer a simple bent 

 tube, as in Fig. 5; it is strongly contracted near the extremities, one of them projecting upwards (»r). 

 At the point of contact of the digestive cavity with the outer wall at m, a second opening has been 

 formed, connecting by a short tube with the pouch of the digestive cavity. Tliis second-formed open- 

 ing (m) is the true mouth, while the first-formed opening (a) now becomes the anus, after having, up 

 to this stage, performed the functions of mouth and anus; end of the second day. 



Fig. 8. The same larva as Fig. 7, seen from above, to show the position of the lobes (ir, tc') formed on 

 each side of the pouch of the digestive cavity ('/), which in Fig. 7 appear like projecting angles (ic). 



Fig. 9. Isolated digestive cavity of a more advanced larva, showing still more plainly the transverse con- 

 tractions of the digestive cavity by which the oesophagus (o), the stomach (d), and intestine (c) are 

 gradually formed, and also the greater projection of the earlets of the pouch, which have become quite 

 elongated laterally ; the opening (o) in the centre is the tube leading to the n.outh. 



Fig 10. The same as Fig. 9, seen in profile; the tube (o) now connects very freely with the mouth (m). 

 formed in the depression, mentioned in Figs. 4, 5, and with the digestive cavity; the currents now 

 change their course, and circulate in the opposite direction. While the larva was in the state repre- 

 sented by Fig. 6, the currents of water enter at the mouth, the future anus (a), circulate in the i)ouch 

 (d), as well as in the earlets formed from the thickening of the wall, and then issued again from the 

 same opening (a). Now the water enters through the mouth (m) (the last -formed opening), pxsses 

 through the'narrow conical tube (o) into the digestive cavity (</), communicating with tl.e earlets 

 (w, t^), and out through the anal opening («), which was the first formed, and furmcrly pedbnned 

 the functions of mouth. 



Fig. 11. Isolated digestive cavity, seen in profile, showing the tube leading from the mouth (m) to the 

 digestive cavity (</), and earlets (w, w'), more developed than i:i Fig. 10. 



Fig. 12. The same seen from above. 



Fi^. 13. Oral end of an isolated digestive cavity, in which the earlets, formed by the pouch, are more 



