314 CHORDATA. 
On the dorsal or upper surface we can notice the paired 
eyes a little way behind the rostrum, and behind them is a 
pair of oval apertures called the sfzracles, for through them 
passes the water employed in respiration. 
The skin is slimy, owing to the secretion of numerous 
slime-glands, and dotted over its surface are numerous 
placeid scales. Each scale consists of a hard base bearing 
a sharp spine. Towards the tip of the tail is a pair of 
small dorsal fins, and the caudal fin surrounds the end of 
the tail. The upper lobe of the caudal fin is larger than 
the lower and contains the true end of the tail. Sucha 
shape of tail is called heterocercal (see Pisces). 
Ventrally we can recognise the median transverse mouth 
with a pair of grooves (07o-nasa/) passing forwards from 
each angle to the olfactory or nasal openings. The jaws 
bear numerous rows of small placoid scales which act as 
teeth. Posterior to each angle of the mouth, and slightly 
outwards, there is on each side a row of five diagonal 
slits leading into the pharynx. These are the pharyngeal 
clefts or gill-sits. Behind the last gill-slit and running 
across the ventral line is the coracoid bar, which can be felt 
through the skin. At the base of the tail is a conspicuous 
median aperture, the cloacal aperture, and close to its 
posterior border is a pair of minute slits, the abdominal 
pores, which lead indirectly into the abdominal cavity. In 
front of the cloacal aperture, and crossing the ventral line, 
may be felt the hard pudic bar. Scattered over the skin, 
especially on the ventral surface, are numerous fine apertures 
of sensory tubes. The tubes are full of a gelatinous material 
which may be squeezed out of the apertures. 
If the mouth be forced open it will be seen to 
lead into a spacious pharynx, into which open dorsally the 
two spiracles and laterally the five pharyngeal clefts on each 
side. Posteriorly it can be seen to taper into an @sophagus. 
If a gill-slit be opened up it will be seen to pass as a 
short canal into the pharynx. On both anterior and 
posterior wall of this canal are a great number of branchial 
filaments constituting the gz//s. In them the blood is only 
separated from the water by a thin membrane and eration 
is effected. On the wall of the spiracle may be noticed a 
pseudobranch or vestigial gill. The water passes in by the 
Alimentary. 
