170 J. B. Johnston 
the gill sac (478), where it runs mesad, to the proper wall of the 
pharynx, turns forward and downward and goes directly beneath the 
base of certain large sense organs which face the pharyngeal cavity. 
Two branches were given off during its course along the base of 
the gill lamellae but all now unite in a rich layer or plexus of fibers 
beneath these organs. In Ammocoetes the gill sacs are large and 
their openings from the pharynx are very broad so that there is 
left between each two sacs only a narrow dorso-ventral strip of the 
proper wall of the pharynx, which is placed obliquely, the dorsal 
end being farther cephalad. The two lateral walls face one another 
closely and leave only a slit-like pharyngeal cavity. Upon these 
areas facing one another are six pairs of sense organs. Each organ 
is a large disc-shaped epithelial structure, thicker in the middle and 
wider along its internal surface (base) than its external surface, and 
somewhat concave superficially. The largest organs extend through 
nine sections and are therefore 90 « in diameter, the medium sized 
ones 60 u. The organs make up the whole thickness of the mucosa, 
rest directly on the connective tissue, and consist of a single layer 
of cells, while the mucosa at either side is many-layered. Two 
kinds of cells are found in the organs: large eylindrical supporting 
cells whose nuclei are at the inner ends of the cells and account for 
the greater width of the base of the organ; and very slender sense 
cells whose nuclei are either at the inner ends or in the middle of 
the cells, and cause a considerable swelling of the cell-body. Whether 
the sense cells have hairs at the surface I could not determine. 
The significance of these organs will be discussed in a later section, 
but here it should be noticed that they are situated on the internal 
surface of the first branchial arch, between the first and second gill 
sacs, and that the ramus visceralis reaches them by way of the 
caudal wall of the first gill sae. That is, the nerve and organs 
lie wholly within the first branchial arch and the nerve is post- 
trematic in position. This is presumably the same ramus which 
HATSCHEK has marked pretrematic in his Fig. 11. It is in reality 
posttrematie as is the nerve to taste buds in fishes. Its position 
will be more clearly understood from Fig. 17 which is a recon- 
struction from the sections on a horizontal plane through the gill 
slits. The glossopharyngeus is wholly posttrematie except those 
numerous twigs mentioned above which go to the mucosa near the 
dorsal surface of the gill sac. These may and probably do supply 
the gill lamellae of the cephalie wall of the gill sac, and if so are 
