314 CHORD AT A. 



On the dorsal or up[.er surface we can notice the paired 

 eyes a Httle way behind the rostrum, and behind them is a 

 pair of oval apertures called the spiracles^ 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 

 placoid 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. Such a 

 shape of tail is called heterocercal (see Pisces). 



Ventrally we can recognise the median transverse mouth 

 with a pair of grooves {pro-nasal) 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-slits. 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 pubic 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 oesophagus. 



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 gills. In them the blood is only 

 separated from the water by a thin membrane and aeration 

 is effected. On the wall of the spiracle may be noticed a 

 pseudnbrajich or vestigial gill. The water passes in by the 



