A DEGENERATE GLAND. — PRINCE. 215 



of fish wholly degenerate later. In the pike-perch or pickerel 

 (SHzostedion vitreuTn) exhibiting when adult a ductless swim- 

 bladder, still possesses the duct when the fish is 6 or 8 inches 

 long, and it is even then hollow for a great part of its length. 

 In the familiar stickleback (Gastrosteics) the duct persists and 

 remains open for a comparatively long time, although in the 

 adult stage it disappears. 



I have made reference to the salivary glands in speaking 

 of the glandular character suggested by the early features of 

 of the swim-bladder. Fishes exhibit no salivary structures 

 whatever, unless the lingual follicles in the Lamprey be of that 

 nature ; but if, as I think it is clear, or at any rate not impro- 

 bable, that the swim-bladder was primitively a gland, which 

 has lost the glandular function, then its function cannot have 

 been remotely unlike a secreting organ, active in providing a 

 medium for lubricating food in the anterior portion of the 

 alimentary canal. Such lubrication of the food became of 

 course unnecessary in fishes, such as the Sharks which practically 

 possess no oesophagus for the huge stomach opens directly to 

 the mouth and the food is gulped at once into that capacious 

 digestive chamber, the mouth with its array of teeth and the 

 wide gullet have chiefly the task of preventing the escape of 

 the seized prey. There are many glands in Vertebrates now 

 without use or whose use is difficult to understand yet they 

 persist. The small finger-like pocket in the rectum of Sharks 

 (the rectal gland) is not understood. In the Chiraaera3 or 

 Rabbit-fishes, closely allied to the sharks, it has degenerated, 

 and forms merely a slight projection on the intestinal wall. A 

 still more remarkable case is the thyroid gland in man, the 

 origin of which must be sought not in the fishes, but in the 

 still lower and more primitive Urochordata or Tunicates. The 

 hypobranchial groove or endostyle of the Ascidians, whose 

 cellular ridge, on the internal ventral wall of the pharynx, 

 secretes a mucus which entangling the particles of food, is 

 passed, by the dorsal lamina, to the digestive tract. The 



