46 



Morphology of the mouth in Phoxinus is relatively simple. All Phoxinus species have a 

 terminal or slightly subterminal mouth; the lower jaw is equal to, or slightly shorter than 

 the upper one. No barbel is present (Fig. 19 A-C). In P. oreas and tennesseensis, the mouth 

 angle is small (<55°) (TS 1[0]), so the mouth gape is horizontal. In other species, the 

 mouth angle is large (>60°) (TS 1[1]) (Fig.l9B), and the gape is slightly oblique. In P. 

 neogaeus, the mouth gape extends to below the middle of the eye's pupil (TS 2[1]); in 

 other species of the genus, however, the gape never reaches the middle of the pupil (TS 

 2[0]). In the outgroups, the mouth angle is small (<55°), and the mouth gape never ex- 

 tends to below the middle of the eye pupil. 



The mouth angle and the gape length increase during growth in species of Phoxinus. For 

 instance, in P. neogaeus, in a 20 mm standard length specimen, the mouth angle is small 

 and the gape is short (almost reaching the anterior margin of eye); in a 55.4 mm standard 

 length specimen, the angle is much larger and the mouth gape reaches middle of the pu- 

 pil (Fig.l9B, C). Phillips (1969a) showed the mouth of P. erythrogaster was less oblique 

 than that of P. eos. 



Olfactory organ (Figs. 20A-B, 21A-D) 



Olfactory organ is a chemical receptor in fish, which can respond to chemical stimuli other 

 than the qualities of sweet, bitter, salty and acid (Harder 1975). In all cyprinids, two na- 

 sal openings (anterior and posterior ones) are present on each side of the head. The ante- 

 rior nasal opening is generally smaller than the posterior one. A nasal bridge is present 

 between the anterior and posterior openings. In Phoxinus, the anterior opening is smaller 

 and forms a short ellipse, and the posterior one is an elongated ellipse. No variation in 

 shape of the nasal openings is observed in the genus Phoxinus. 



A erect structure (nasal septum) is present from dorsal side of the bridge (Fig.20A, B). 

 This nasal septum is an extension of the skin on the bridge. In P. phoxinus, brachyurus, 

 and neogaeus, the dorsal side of the nasal septum is deeply concave (TS 3[1], Fig.20A); 

 whereas, in P. erythrogaster, cumberlandensis, tennesseensis, oreas, eos, and issykkulen- 

 sis, the nasal septum is not concave on its dorsal side (TS 3[0], Fig.20B) - a similar con- 

 dition found in the outgroups. 



The entire margin of the anterior nasal opening is erect (TS 4[0]) in most Phoxinus spe- 

 cies and in the outgroups. In P. erythrogaster, the middle of the anterior margin is not er- 

 ect (TS 4[1], Fig.20B). 



Similar to the condition in most cyprinids, the olfactory organ in Phoxinus is composed 

 of primary lamellae and axis (raphe). No secondary lamellae are present in Phoxinus. (In 

 some cyprinids, e.g., Parabramis pekinensis, secondary lamellae are present on the pri- 

 mary ones - see Chen 1988b.) Numerous melanophores are present on both primary la- 

 mellae and axis. 



Morphology of the axis and number of the primary lamellae vary ontogenetically. For in- 

 stance, in P. neogaeus, the axis of the olfactory organ is very short and located at the an- 

 terior part of the organ, five primary lamellae are present in a 25 mm standard length 

 individual (Fig.21D). In a 30 mm standard length individual, the axis is more elongated, 

 almost reaching the middle of the organ, six primary lamellae are present. The axis is 



