MEM. M. C. Z., IX. No. 2.— ECHINODEEMATA. 23 



16. Twenty-two hours after fecundation ; the embryo has become greatly lengthened, and is cylindrical. The cavity 

 (d) has slightly expanded at the closed extremity, and the walls of the embryo are somewhat reduced in thick- 

 ness except at the perforated region ; the body is .'Somewhat translucent, slightly tinged with ochre-color. 

 The opening («) still serves as a mouth, although in more advanced stages a second opening is formed 

 which becomes the true mouth of the enibrj'o, at which time the present muulh then becomea the anus. 



ir. Slightly older embryo than lig. 16, seen from the side ; the digestive cavity (</) is no longer in the axis, but 

 is bent to one side (the lower side). 



18. Older embryo, seen in profile ; the pouch formed at the end of the closed extremity of the bent digestive cav- 



ity (d) is nearer the lower side than in fig. 17, and is moving towards the slight depression (»i, the future 

 mouth) placed in the middle of the larva. 



19. A larva somewhat more advanced, seen in prolUe, in which the terminal ponch of tlic digestive cavity has 



actually come into contact with the wall of the lower side at m. The dorsal region of llic jicrforatcd 

 extremity projects slightly beyond the depression in which m is placed. The digestive cavity is not yet 

 divided into distinct regions. 



20. A larva somewhat more advanced (end of the second day), seen in profile. The digestive cavity is no longer a 



simple bent tube, as in fig. 18 ; it is strongly contracted near the extremities ; at the distal extremity two 

 diverticula have formed, projecting upwards («') ; a second opening (m ) has been formed at the point of con- 

 tact of the former closed extremity of the digestive cavity with the lower side; this connects the (esoph- 

 agus, by a short tube, with the main pouch of the digestive cavity. This second formed opening {iii) is the 

 true larval month, while the first formed opening (a) now becomes the anus, after having, up to this stage, 

 performed the functions of both mouth and anus. 



21. Isolated digestive cavity seen from below, showing the position of the mouth and anus on the same side of the 



larva. The aual extremity of tlie larva bending over as in fig. 24 at about this stage, thus bringing the 

 anal opening fi'om the extremity of the larva to the lower side. The two diverticula («-• w') of the digestive 

 cavity (the future water-tubes) are so far differentiated as to be quite distinct from the digestive cavity. 

 The walls of these diverticula are excessively attenuated, and are scarcely connected with the digestive 

 cavity. 



22. Larva somewhat older than stage of fig. 21, seen from above, in which the two small bodies, ir, «', the diverti- 



cula of younger stages formed from the pouch of the digestive cavity at its closed extremity (the problem- 

 atic bodies of Miillcr), have entirely separated from the digestive cavity from wdiich they weie formed ; the 

 three divisions of the original cavity into intestine, stomach, and resophagus are plainly markeil out. 



23. Older larva, seen from below at the end of the third day after fecundation, showing the triangular shape of 



the mouth (m), the greater size of the problematic bodies w, w' (the water-tubes), wliieh increase inde- 

 pendently and at an unequal rate ; the tube tc' communicates with the madreporic opening (?)) ; it also 

 shows the position of the rudimentary oral and anal vibratile crescent cords. 



24. The same as fig. 23, seen in profile, to show the position of the mouth in a strongly marked depression, the 



gieat increase in size of the oral part of the oesophagus, the swelling of the stomach, and the bending of 

 the extremity of the intestine back and downward toward the mouth, so as to make a small angle with 

 the trend of the stomach. 



25. Slightly older larva, seen from above. The principal difference between this st.age and the preceding one con- 



sists iu the greater increase in size of the vibratile crescents, which now form two small plastrons, and the 

 greater size of the water-tubes. The intestine also bends so as to make, when seen in profile, almost a 

 right angle with the stomach, which is piished out farther toward the anal extremity. 



26. More advanced larva, seen from the left profile, in which the oral pouch has assumed its characteristic 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 aspect it closely resembles a retort, the stomach being the receiver, the 

 intestine the tube. The anal and oral vibratile crescents are greatly extended towards the extremity of the 

 body, the one on the oral, the other on the dorsal side. 



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 dorsal water-pore (madreporic body) to the water-tube (»■') is quite 

 distinctly seen. 



28. The same larva as fig. 27, seen from below, showing the intestine thrown to one side of the axis of the larva, 



the water-tubes extending along the sides of the stomach toward the anal extremity. 



29-34. Asteracantkion prdlidus. From Alexander Agassiz, Embryology of the Starfish, 1864 (Agass. Cont. Nat. 

 Hist. U. S., v.). Pis, III., IV., VII. Memoirs Mus. Comp. Zool., 1877, V., No. 1, North American Star- 

 fishes, Pis. III., IV., VII. 



29. Larva seen from the right profile, somewhat more advanced than any larva of A. bcryiimm raised by artificial 



fecundation. 



30. The same larva seen from the oral side. The water-tubes have greatly increased in diameter ; they have 



united beyond the mouth, and also extend along the sides of the stomach so as to meet, but without unit- 



