PULMONARY ORGANS OF FISHES. 79 



meshes of which scarcely exceed the diameter of the capillaries 

 themselves. These are covered by a single layer of large flat 

 epithelial cells, from which, as in Amphibia and Reptiles, short 

 nucleated processes are given off into the capillary meshes. 



The swimming-bladder of Fishes, notwithstanding that it is 

 a pure hydrostatic apparatus, destitute of any respiratory 

 capillaries, closely approximates the lungs in a morphological 

 point of view. 



The microscopic characters of the tissues entering into its 

 composition are as various as the appearances it presents to 

 the unaided eye, being sometimes simple, sometimes divided 

 by constrictions or possessing diverticula, sometimes having 

 walls that are perfectly smooth, and in other cases again with 

 septa and ridges projecting from the inner surface. The 

 principal constituent is an external layer of compact connective 

 tissue, lying immediately beneath the peritoneum, composed of 

 long, fine, and in many osseous fishes, peculiarly stiff connective- 

 tissue fibrils that sometimes run collectively in the same trans- 

 verse or oblique direction, and sometimes form decussating 

 fasciculi, being then frequently arranged in two completely 

 distinct layers, an external longitudinal and an internal 

 transverse. 



A process of ossification sometimes occurs in this external 

 fibrous layer, as in Cobitis fossilis, Acanthopsis and Ophidium 

 imberbe ; in Cobitis assuming the form of a continuous trellis- 

 work with rounded meshes. 



The loose stroma of fibrillar connective tissue, which may be 

 regarded as the internal layer, frequently contains elastic 

 lamellae lying parallel to the surface. In the greater number 

 of osseous fishes these are remarkably delicate; in a few, 

 however, and especially in the anterior part of the swimming 

 bladder of the Cyprinoid, they acquire the character of a dense 

 fenestrated membrane. Peculiar elongated, four-sided, delicate 

 elastic leaflets frequently appear between the elastic lamellae ; 

 as, for example, in Esox lucius, Perca fluviatilis, Gadus callarias, 

 Gadus zota, etc., which are perfectly transparent and structure- 

 less up to the oval nucleus, usually situated near the middle, 

 and when isolated roll up like a leaf. These leaflets usually 

 lie superimposed on each other in small packets, but can be 



