128 GEORGE W. FIELD. 



PLATE XV. 



Fig. 24;. — A Bipiuuaria, a little older than Fig. 28, seeu from the right 

 side. Kleiaenberg's picro-sulphuric cedar ; oil preparation. X 70, 



Fig. 25. — Surface view of the tip of the preoral lobe of a four days old 

 larva, before the division of the circumoral ciliated band into the two bands 

 characteristic of the Bipinnaria. 



Fig. 26. — The same view of a larva six days old, after the division of the 

 circumoral band. 



Fig. 27. — Part of a transverse section of a Bipinnaria, a little older than 

 that shown in Fig. 24, showing a later stage of the schizocoel {s. v.) and its 

 relative position. Pereuyi ; Delafeld's hsematoxylin preparation, x COO. 



Fig. 28. — A stage a little older than Fig. 30. The characteristic arms have 

 begun to form. The enterocoels have united in the preoral lobes, but the 

 cavities have not yet become continuous. From the living specimen, viewed 

 from the dorsal surface, x 70. 



Fig. 29. — A Bipinnaria about live weeks old, seeu from the ventral surface. 

 Kleinenberg's picro-sulphuric ; alcoholic borax-carmine preparation. X 70. 



Fig. 30. — Bipinnaria about eighteen days old, seen in ventral view. Shows 

 the formation of the preoral ventral area (p. v. a.), and the position and form 

 of the enterocoels. From the living specimen. X 70. 



Fig. 31. — Portion of a longitudinal section parallel to the dorsal surface, 

 from a Bipinnaria of about the same stage as Fig. 29, to show a later stage of 

 the schizocoel. Kleinenberg's picro-sulphuric, Delafeld's hsematoxylin pre- 

 paration. X 600. 



Fig. 32. — The growing anterior tips of the enterocoels, drawn from the 

 living specimen. Same stage as Fig. 30. x 240. 



Fig. 33. — The union of the enterocoels in the preoral lobe. Same stage as 

 Fig. 28. From the living specimen, x 240. 



Fig, 34:. — A transverse section of a Bipinnaria at about the same stage as 

 Fig. 24, at a point just anterior to the water-pore (a tangential piece is 

 cut from the pore canal, w- (■), to show the origin of the schizocoel, *. v. 

 Kleinenberg's picro-sulphuric, Kleinenberg's hematoxylin preparation. X 240. 



