THE SENSORY CANAL SYSTEM OF FISHES. 518 
of cluster pores. There is no connection between the two sides 
(P1. 39, fig. 1). 
4, Tur Ciuster PorEs.—Distributed over the head and 
along the sides of the body are a series of organs which I have 
termed cluster pores. They are subject to great variation in 
both number, location, and form—a fact which seems to have 
been lost sight of by many observers. 
On a specimen of Polyodon, 1660 mm. in length, there 
were counted 132 series or patches of these pores, distributed 
as follows : 
On each side of the head : . 12 = 94 
On the dorsal surface of the head . 387 7=)" 38 
On each side of the body ‘ woont—— WAU 
132 
Each series of pores arises as a small single branch from the 
lateral or main canal, or one of its chief branches ; it traverses 
the surface for a short distance and then divides into from 
three to twelve still smaller branches, each of which opens to the 
surface by a small oval or circular pore, the peripheral border 
of which is pigmented. In a few cases division of the terminal 
branches was noted, the pores assuming the form of the figure 8. 
Fig. 6d (Pl. 39) illustrates one taken from the lateral canal. 
In longitudinal and transverse sections the following struc- 
ture was noted. They agree almost in every detail with the 
“spalt-papillen” of Fritsch. The neck is surrounded by a 
layer of epithelial cells, at the base of which a series of 
columnar supporting cells—whose nuclei stain deeply—sur- 
round a series of pyriform sense-cells containing large oval 
nuclei at their inner and lower ends and hair-like processes at 
the opposite ends. The supporting cells are surrounded by the 
epidermis. At the base of the layers of columnar epithelium 
nerve branches pass, which break up into a series of fine 
terminal fibres and pass into the sense-cells (Pl. 39, fig. 4). 
5. THE PrimitivE Pores.—These are strictly confined to 
the region of the head and rostrum. In Polyodon they are 
developed to an enormous extent, covering the whole of the 
