216 THE SWIM-BLADDER OF FISHES 



endosfcyle has an alimentary function and is found in the 

 Enteropneusta (Balanoglossiis). It is represented in the larval 

 lamprey, and in Amphioxus, an d, indeed, in some stages, in all 

 Craniota. As the tongue, in the higher forms, becomes more 

 important and grows, the endostyle is reduced and assumes the 

 character of a canal closed off from the mouth cavit3^ It 

 persists as the thyroid gland in the high est Vertebrates, indeed 

 in man himself, but its original physiological I'elations and 

 purposes are lost, and it becomes wholly disconnected with the 

 digestive canal. As Dr. T. M. Studler (No. 20) has shown 

 the right thyroid rudiment, in a 5 J weeks embrys (Homo), is 

 still connected with the pharynx ; but its fellow on the left is 

 further advanced and has lost its primitive connection.* 



The swim-bladder is an outgrowth of the stomodaeum or 

 rather the posterior buccal portion of the alimentary canal, the 

 part which in a great majority of animals is very well provided 

 with glands. If it be, as I have ventured to suggest, a 

 degenerate gland, its anterior position is explained, much as its 

 connection may shift later in obedience to physiological and 

 anatomical exigencies, ceasing, indeed, to have any oesophageal 

 connection whatever in the physoclistous fishes. In such a fish 

 as the herring, it does not communicate with the pharynx, but 

 with the gizzard-like crop, while its posterior attenuated continu- 

 ation opens by a duct on the left side of the anus (Plate I, fig. 3, x.) 

 Its persistence in so many fishes, though it has disappeared in 

 so many, may be explained by its deep position. An organ 

 removed from the external modifying conditions, which may not 

 not merely reduce but obliterate useless organs comparatively 

 rapidly, do not affect so potently a deeply -seated useless organ 

 and the tendency is for such an organ not to disappear but to 

 be modified for a variety of functions. It is easy to understand, 

 therefore, not only its persistence, but its actual increase in size 

 and complexity in spite of its increasingly unimportant and 



*0ther glands might be referred to, such as the thymus which is largest in man 

 two years after birth and diminishes with age, w^hile the thyroid, which is largest at 

 birth, may wholly disappear in the adult. 



