314 MR. J. E. DUEEDEN ON 



Plate 18. 



Fit's. 1-7 are not drawn to any scale ; 1-6 are representations oP the living 

 larvis as seen under a low power of the microscope, and 7 as seen with 

 a hand-lens. All the others are from sections, and were drawn with 

 tlie assistance of a camera. 



Fig. 1. The usual form of the freshly-extruded, free-swimming larva. 



Fig. 2. A form occasionally assumed. 



Fig. 3. The contracted cake-like condition. 



Fig. 4. Oral aspect of a free-swimming larva a few hours after extrusion. 



Fi". 4 a. Lateral aspect of a lai'va a few hours after extrusion. The tenta- 

 cular protuberances are beginning to appear. 



Fig. 5. Oral aspect of free-swimming larva. 



Fig. 6. The same seen from the side. 



Fio-. 7. A larva three or four days old. (See Plate 19.) 



Fig. 8. Vertical section through the middle region of the column-wall of a 

 non-extruded larva in which the central tissue has already become 

 disorganized. X 450. 



Figs. 9-11. Vertical sections through the archenteric region of a larva very 

 shortly after liberation, showing the formation of the oral aperture and 

 oesophagus. X 320. 



Fig. 9. Tangential section near the periphery of the archenteric wall. Tlie 

 flattened inturned ectoderm is in close contact witli the flattened 

 archenteric wall below. The upper narrow slit indicates the upper 

 outer surface of the floor of the former, and the broad slit below is 

 the central ceelomic space, to the right continued peripherally and 

 below. 



Fig. 10. Taiigential section near the centre. The actual oral aperture appears ; 

 the floor of the invaginated -ectoderm is convex and produced 

 laterally. 



Fi". 11. Radial section. The floor of the invagination and the roof of the 

 archentei'on are here broken through and complete communication is 

 established between the interior and -exterior. The invaginated 

 ectoderm forms the greater part of the oesophageal wall, but the 

 innermost portion is derived from the archenteric eudoderm. The 

 union between the two is indistinguishable, but from other sections 

 it is clear that it occurs about halfway below the lateral inden- 

 tations. 



Figs. 12-14. Transverse sections of a freshly-extruded larva before dis- 

 organization is set up. X 250. 

 Fig. 12. Through the upper archenteric region. The section is taken slightly 

 obliquely to the vertical axis and serves to represent the various 



