830 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



The posterior part of the intestine, from the beginning of the 

 spiral valve to the anus, is connected with the ventral wall of the 

 abdomen by a mesentery. 



The air-bladder (a.b.} is 45 centims. long, and opens into the 

 alimentary canal by a slit-like aperture (a.fr.) on the median 

 dorsal line, immediately behind the epipharyngeal teeth. Each 

 lip of this aperture is largely formed by a muscular cushion, 

 thickest at its posterior end, and extending about 6 millims. 

 behind the aperture itself. A narrow passage is bounded by 

 these muscular walls, which opens dorsally into the air-bladder. 



The air-bladder is provided with two short anterior cornua, 

 and tapers to a point behind : it shews no indication of any 

 separation into two parts. A strong band of connective tissue 

 runs along the inner aspect of its whole dorsal region, from 

 which there are given off on each side at intervals of about 

 12 millims. anteriorly, gradually increasing to 18 millims. pos- 

 teriorly bands of muscle, which pass outwards towards its side 

 walls, and then spread out into the numerous reticulations with 

 which the air-bladder is lined throughout. By the contraction 

 of these muscles the cavity of the air-bladder can doubtless be 

 very much diminished. 



The main muscular bands circumscribe a series of more or 

 less complete chambers, which were about twenty-seven in 

 number on each side in our example. The chambers are con- 

 fined to the sides, so that there is a continuous cavity running 

 through the central part of the organ. The whole organ has the 

 characteristic structure of a simple lung. 



The liver (lr.} consists of a single elongated lobe, about 32 

 centims. long, tapering anteriorly and posteriorly, the anterior 

 half being on the average twice as thick as the posterior half. 

 The gall-bladder (g.b.} lies at its posterior end, and is of con- 

 siderable size, tapering gradually so as to pass insensibly into 

 the bile duct. The hepatic duct (kp.d) opens into the gall- 

 bladder at its anterior end. 



The spleen (s.) is a large, compact, double gland, one lobe 

 lying in the turn of the intestine immediately above the spiral 

 valve, and the other on the opposite side of the intestine, so that 

 the intestine is nearly embraced between the two lobes. 



