510 WALTER EDWARD COLLINGE. 
dorsal and twenty-two ventral, fourteen of the twenty-two 
being situated on the dorsal caudal fin. The number and 
position may vary on either side of the body. None of the 
branches are of any great length, ranging from 12 to 17 mm. 
Kach branch terminates in from four to seven still smaller 
branches opening to the surface by a series of pores (Pl. 39, 
fig. 2,2. c. b.). There are seldom less than two and rarely 
more than twelve in number. These are the cluster pores. 
Along the whole length of the canal and its branches are two 
series of sensory organs, about which, from the condition my 
specimens were in for histological purposes, I can say but little. 
The first are a series, sixteen to twenty in number, of fairly 
large circular markings—3}$ to 4 mm. across—slightly raised 
above the epidermis. They are distributed at irregular in- 
tervals along the canal and open to the surface by a transverse 
aperture or slit. Sections cut by a freezing microtome showed 
them to consist of a layer of epithelial cells, with numerous 
goblet cells forming a follicular-shaped organ. At the base of 
the follicle a sensory organ was present, in which no difference 
could be seen trom those found in the cluster and primitive 
pores, excepting in size. From their structure and position I 
regard them as modified cluster pores and synonymous with 
the sensory follicles which Fritsch (29) speaks of as “ spalt- 
papillen.” Externally they are very like the “ seitenorgane ” 
figured by Leydig (49, Taf. viii, fig. 13, 0.) in Petro- 
myzon. . 
The second series are much more numerous. They are dis- 
tributed over the canal and its branches, and also in the imme- 
diate neighbourhood. ‘They are present in the greatest number 
at the posterior end of the body. ‘There are from 800 to 1000 
on each side of the body. They are very small, the largest not 
measuring more than 8 mm. across. They appear as a small 
ring enclosing a depression in the centre of which arises a 
-small, hard, conical body, opening at the apex by a minute 
slit-like pore. In one or two cases they were observed in 
groups (Pl. 39, fig. 6, 6 and c); these were not on the canal, 
but in close proximity. Those upon either the canal or its 
