540 
EDWIN S. GOODRICH. 
Although the pit and its lining epithelium have been 
described by several authors — Hatschek ( 6 ), Langerhans ( 8 ), 
Willey ( 16 ), van Wijhe ( 14 ), Andrews ( 1 ) — the complexity of 
its histological elements seems to have escaped the notice of 
these observers, and its finer structure deserves further study. 
In the adult the epithelium is composed of cells roughly 
disposed in three layers (PL 28, figs. 12, 13, 14). The most 
superficial cells are large, with a broad end reaching to the 
Text-fig. 1. 
Left side-view of the head of a young Amphioxus. The left 
body-wall, oral hood, and wall of the pharynx have been 
cut away, exposing the right half of the wheel-organ, w.o. 
Hatschek’s pit, H.p., is seen by transparency, n.c. nerve 
cord. n.p. Olfactory pit. n.t. Notochord, o.h. right oral 
hood. pli . Pharynx, v. Yelum surrounding the true mouth. 
free surface, and bearing a bunch of fine cilia generally 
gathered together to form a flame-like tuft. Their nuclei are- 
pale and rounded. The middle layer is composed of narrower 
cells, with oval, deeply staining nuclei. Each of these cells is 
prolonged to the surface and beyond it into a long, narrow, 
stiff, rod-like extremity bearing a single stout cilium. The 
third and simplest variety of cell forms the deepest layer next 
to the basal membrane covering the organ. The peculiar 
rod-bearing cells are arranged in four or five transverse rows- 
