332 A HISTORY OF FISHES 



adult. First the eyes appear, the mouth is contracted and 

 takes on the suctorial disc-like form so characteristic of the 

 Lampreys, the tongue and horny teeth are developed, and the 

 branchial groove disappears. At the same time important 

 changes take place in the form of the skeleton, gill-pouches, 

 alimentary canal, kidneys, etc., the whole process occupying 

 about two months. 



Among other examples of larval or provisional organs, the 

 adhesive or cement organs and the external gills may be 

 mentioned. In the Lung-fishes (Dipneusti) the newly hatched 

 larva is not unlike the tadpole larva of the amphibian : there is 

 no yolk-sac, the small amount of food-material still present 

 being distributed over the lower region of the body. The 

 resemblance is further strengthened in the African (Protopterus) 

 and South American (Lepidosiren) forms by the presence of 

 four pairs of feathery external gills, projecting freely from the 

 gill-arches, and of a glandular adhesive organ situated behind 

 the mouth. In the South American species the gills disappear 

 during the metamorphosis (Fig. iqa), being replaced by the 

 internal gills and lungs, but in the African species vestiges are 

 retained throughout life (Fig. 99A). In the larval Bichir 

 (Polypterus) there is a single pair of fringed pinnate external 

 gills (Fig. 19B), which generally disappear completely during 

 the metamorphosis, but occasionally one or both are retained 

 for a further period. Some others of the more primitive Bony 

 Fishes have been described as possessing larval external gills 

 {Mormyridae, Osteoglossidae, Cobitidae, etc.), but these are merely 

 the ordinary gill-filaments which are excessively long and 

 prolonged to the exterior. The development of these structures 

 is always correlated with life in badly aerated, tropical swamps, 

 and there can be little doubt that they assist the respiration 

 of the larvae until the permanent breathing organs are de- 

 veloped. The larval Bow-fins [Amia) or Gar Pikes (Lepidosteus) , 

 hatched in the more temperate climate of North America, and 

 in better aerated water, do not possess external gills. They 

 are, however, provided with cement organs, but instead of being 

 placed behind the mouth, these are situated at the end of the 

 snout, a position occupied by similar organs in the larval 

 Ascidian. Traces of the sucking disc are to be found in the 

 larval Sturgeon {Acipenser), in the form of a shallow pigmented 

 groove in front of the mouth, but such a structure is never 

 present in the larvae of any of the higher Bony Fishes. Its 

 function is to enable the larvae to attach themselves to weeds 



