iç)i6.] Fauna of the Chilka Lake : Larval Fish. 389 



Stage I. This is the smallest specimen I have come across. It measures 25 

 mm. in length. I have not sketched it, as the poor fixation of this specimen makes 

 it difficult to determine the details of internal anatomy with any degree of accuracy. 

 The eye and the gas-bladder is already formed. The lower jaw is prolonged into a 

 slender beak. On the ventral surface a big chromatophore is present (see pi. xviii, 

 fig. 1). The dorsal surface of the gas-bladder is pigmented. The tail is proto- 

 cercal. 



Stage II. This specimen measures 275 mm. in length. The beak or snout 

 becomes slightly more elongated and distinctly pronounced at this stage. The same 

 chromatophore is present on the ventral surface. The tail begins to assume the 

 heterocercal type, and the notochord and myotomes become fairly distinct. The 

 cartilaginous development of the skull and visceral arch has begun. The cartilage 

 cells are quite distinct. 



Stage III (pi. xviii, figs. 1 and 2). The eye is well developed and looks like an 

 opaque black mass. Dorso-ventrally it is longer than laterally. The beak is curiously 

 shaped, being bent in front like that of some birds. The visceral arches are well 

 developed. Under the high power of the microscope, the visceral arches are seen to 

 be lined by more or less parallel rows of cartilage cells. The heart lies just below and 

 behind the basibranchial cartilage, and is in a fairly advanced stage of development. 

 The various chambers are however just formed, and their connections and the various 

 blood vessels which they give rise to are quite indistinct. The gas-bladder has an oval 

 shape and is invested dorsally with pigment bodies. It is continued anteriorly into a 

 hollow tube-like structure which seems to open just at the junction of the pharynx with 

 the oesophagus. The notochord is well developed, and bends sharply upwards at the 

 caudal end to form the beginning of the heterocercal type of tail fin. The myotomes 

 and myocommas are developed, but the body is still more or less transparent. The 

 alimenta^ canal is formed, but its different regions are rather indistinct. It lies close 

 beneath the gas-bladder. Posteriorly it opens by the anal aperture. The glandular 

 epithelium lining the internal cavity of the stomach is visible under the high power 

 of the microscope, but the cavity of the stomach is very narrow. The spinal cord 

 lies just above the notochord. The brain is also formed, but its various regions are 

 indistinct. The liver is also formed and lies beneath the oesophagus and the anterior 

 region of the stomach. It becomes more distinct in the next stage. The big stellate 

 chromatophore still persists on the ventral side of the body. Except for this one, and 

 those in the gas-bladder, no other chromât ophores are to be seen in the body. The 

 dorsal, ventral, and caudal fin-folds are still continuous and quite distinct. 



Stage IV (pi. xviii, fig. 3). The specimen has been sketched exactly as it was 

 found with the mouth wide open. It gives a good idea of the relation of the upper 

 and lower jaws The cartilage cells are very numerous and prominent at this stage. 

 The mandibular, hyoid, and branchial arches are all well developed. The last 

 branchial arch is rather indistinct. Rows of papilla-like outgrowths appear on the 

 first 4 branchial arches, those of the first 3 being quite prominent. These seem to be 

 the rudiments of the branchiae. The opercular membrane also makes its appearance. 



