^joI!ilL!t}^!'iySf\ of the Ovum of the Pike. 201 
siglit and smell ; and (the vessel from i to g being obliterated) ends 
its course in the refluent branch, I, of the first branchial artery. 
This vessel does not enter the branchial, but it terminates in a 
pseudo-branchial apparatus, or false gill, in the situation I. After 
supplying this apparatus the return blood from this false gill is 
collected into a vessel (s), which supplies the eye with blood, and 
hence is called the ophthalmic. Fig. 31 shows diagram matically 
the remarkable change that takes place. 
As circulation becomes well established in the branchiae so that 
respiratory changes can take place to a considerable extent, the sur- 
face of the yelk-bag is less needed as a breathing apparatus ; the 
channels appear to obtain walls of their own; they diminish^ in 
number ; the yelk-bag becomes darker in consequence of numerous 
pigment cells becoming scattered over its surface, and lessens in 
size, the vessels doubtless becoming cutaneous veins, and entering 
the large or cardinal vein, which empties itself into the auricular 
sinus. 
In order to protect the delicate structures of the gill from exter- 
nal injury, a movable covering or flap is placed over it: this is 
called the operculum (the dark line commencing at h, Fig. 32, 
shows the outline). This structure is continually in movement, 
flapping to and fro; and by means of this movement the water 
which is taken into the mouth is, when the mouth is closed, driven 
through the branchial interspaces. The operculum at first is trans- 
parent, and covers the branchial and praecordial spaces and the 
auricular sinus completely. The to-and-fro movement takes place 
twenty-five times in a minute, whilst the beats of the heart are 100. 
It is interesting to notice that the ratio of inspiratory and cardiac 
movements is the same as that of the human subject. In the case of 
man, the respiratory act occurs eighteen times per minute, and the 
cardiac movement seventy- two times, or four times as often, just as 
we find it in the fish. At the edge of the firm operculum is a trans- 
parent, thin, structureless membrane, which also moves to and fro 
with each respiratory efibrt, so that the edge of it describes a curve 
of a quarter of a circle: this is the branchiostegous membrane. 
Behind this, and indeed touched by it, is the pectoral fin (a, Fig. 33, 
which is a view from the back of the fish; it projects outwards). 
This fin is in a state of constant and rapid tremulous motion. The 
Os Hyoides (m, Fig. 32), with its attached branchial arches, moves 
at a joint seen at I. In the first place the mouth is opened, and 
water is taken into its cavity ; at the same time, the hyoid bone, 
with its attached structures, including the operculum, moves down- 
wards, as it necessarily must do when the lower lip is depressed ; 
and the operculum opens widely. The mouth then closes ; by this 
means the hyoid bone is drawn upwards, the cavity of the mouth 
is lessened, and part of the water escapes through the gills, aerating 
