EXPLANATION OF THE PLATES. 



(3) 



peared, twelve hours after fecundation. The depression 

 at a, assumes here somewhat the aspect of a digestive 

 cavity. 



Fig. 26. Seventeen hours after fecundation ; the embryo 

 has lost its spherical shape and has become somewhat 

 pear-shaped; a transverse section is still circular. The 

 depression made by the thickened walls has increased 

 in depth ; the opening (a) performs the functions of a 

 mouth and anus; d indicates the bottom of the diges- 

 tive cavity. 



Fig. 27. Twenty hours after fecundation ;' the depression 

 has the appearance of a small pouch (d) hanging in 

 a pear-shaped body with circular section, showing no 

 deviation from the absolute radiate type ; the opening 

 (a) still performing the double functions of mouth and 

 anus. Currents of water circulate in this cavity, as 

 they would in the digestive cavity of any Polyp or 

 Acaleph in about the same stage 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 in a trans- 

 verse section. The cavity (d) has slightly expanded 

 at the 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 somewhat translucent, and slightly tinged with 

 ochre color. The 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, 10, 11, 12, 13, 14, 15, 16, 17, the diges- 

 tive 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. 1 seen in profile ; the cav- 

 ity 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 pro- 

 file, in which the pouch has actually come in contact 

 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 the 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 epithelial cells 

 have appeared on the surface. 



Fig. 7. A somewhat more advanced larva, seen in pro- 

 file ; 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 (w). 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. This second-formed opening (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 (w, w>) formed on each 

 side of the pouch of the digestive cavity (d), which, 

 in Fig. 7, appear like projecting angles (•«;). 



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 oesoph- 

 agus (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 elon- 

 gated laterally; the opening (o) in the centre is the 

 tube leading to the mouth. 



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 represented by Fig. 6, 

 the currents of water enter at the mouth, the future 

 anus (a), circulate in the pouch (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 («) (the. 



