xliv AIR-BLADDER 



octocrane containing the sac of the great otolite" (Owen). It has likewise 

 been remarked that in the scad or horse-mackerel, Caranx traehnrus, a 

 canal passes from the air-bladder to the bronchial cavity, permitting the 

 escape of air, although it does not serve to admit it.* Knert observed that 

 in several fishes provided with pectoral pores, the thymus gland is absent, 

 and the air-bladder communicates with the oesophagus by an open duct in 

 some Acanthopterygians, as Holocentrwm, Priacanthus, Gcesio, &c. The 

 air-bladder may have lateral attachments, as in Scicena, or blind appendages, 

 as in Polynemus. The interior of the air-bladder of Physoclisti, as the cod, 

 is lined with a thin membrane of silvery whiteness, composed of a series 

 of fibres, covered with a basement membrane, provided with scales of 

 epithelium ; beneath this is a layer of vessels, while inside is situated a 

 highly vascular body, receiving blood direct from the aorta : capillaries 

 exist, and^ here veins commence. In the perch the glandular body is not 

 in one compact mass, but scattered about its interior. J 



If we examine the Pliysostomi, or those families in which a pervious 

 pneumatic duct exists throughout life, we find the majority of such are 

 fresh-water forms, situated between the Physoclisti on one hand and the 

 Dipnoids and Ganoids on the other. This pneumatic tube possesses the 

 same coats as the air-bladder, is of various lengths, and is said to be 

 occasionally tortuous. As a rule it opens upon the dorsal surface of the 

 alimentary canal, but in some of the herring family directly into the 

 stomach. The glandular body observed upon as existing inside the air- 

 bladders of the Physoclisti, is not so well developed, as a rule || in the 

 Physostomi. As this pneumatic tube has no muscular coat its diameter 

 can hardly vary, except when acted upon by other forces, it would therefore 

 be useless for inspiration. If we examine a carp we find its air-bladder is 

 generally a simple sac, with a constriction between its anterior third and 

 posterior two-thirds, but not sufficient to close the communication. From 

 the posterior portion of the air-bladder^f springs the pneumatic tube, as 

 already described; or else this organ may be in the form of two rounded 

 portions placed side by side beneath the bodies of some of the anterior - 

 vertebras, and not communicating with each other; but the two pneumatic 



* A. Moreau, Compt. Rend., Ixxx, pp. 1247-1250. 



t Sitz. Ak. Wiss. Wien. 1864, xlix, May, pp. 455-459. 



% Quekett, Trans. Microp. Soc, i, 1844, p. 100. 



The majority of marine Physostomi are littoral, or surface swimmers, often weak forms, 

 which have to escape pursuit of enemies while rapidly rising' to the surface. 



| In the eel there are two, placed one on each side of the duct communicating between its two 

 portions: the distribution of its vessels in the upper cpmpartment of its air-bladder Quekett 

 likened to the cellular lung of reptiles. 



^1 In carps the anterior portion, of the air-bladder is very elastic, the posterior hut slightly so. 

 Miller observed that "in proportion as the fish rises in the water the anterior bladder, which is 

 the most elastic, ninsL considerably increase in volume, and thus keep ibe head of the animal up, 

 while the contrary must be the case when the fish descends." 



