1 62 ELEMENTARY BIOLOGY. [CHAP. 



Examine from the side. 

 The head ; well marked and possessed of func- 

 tional sense organs. 



j3. The external branchia ; long and pectinated; 

 three in number on either side, a small third- 

 pair having appeared. 



y. The tail ; elongated and marked out into a 

 number of well-defined segments (myomeres). 



8. The median fin ; a thin fold of integument en- 

 circling the tail and continued forwards — dor- 

 sally to the middle of the back — ventrally, for 

 a short distance in front of the anus (cloacal 

 orifice). 

 Examine from beneath. 



c The mouth; transversely enlarged and sur- 

 rounded by fleshy lips, within which can be seen 

 the jaws, beset by small horny teeth. 



/. The larva, at a later period. Look for larvae in 

 which the external branchiae of the right side are 

 no longer visible. Examine from beneath, 

 a. Head, trunk and tail. Cf. generally with the 



foregoing stages, as to relative proportions. 

 /3. The external branchice ; visible only on the left 

 side, projecting out from beneath a fold of the 

 cephalic integument (operculum). A similar fold 

 is seen on the opposite side, 

 y. The mouth; transversely oval and very large; 

 the lips, now papillate ; horny teeth, more 

 marked. 

 8. The suckers; disappearing, and reduced to the 

 condition of a couple of small tubercles. 



