758 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



seen to leave a large circular opening leading to the gill-arches, 

 into which a current of water is constantly passing, by the lateral 

 expansion and contraction of the gill-cover; the outer extremity 

 of the gill-cover covers the base of the pectorals. In a some- 

 what older stage the snout has become more elongated, the 

 sucker more concentrated, and the disproportionate size of the 

 terminal sucking-disc is reduced ; the head, when seen from 

 above, becoming slightly elongated and pointed." 



In a larva of about 18 days old and 21 millims. in length, of 

 which we have not given a figure, the snout has grown greatly 

 in length, carrying with it the nasal organs, the openings of 

 which now appear to be divided into two parts. The suctorial 

 disc is still a prominent structure at the end of the snout. The 

 lower jaw has elongated correspondingly with the upper, so that 

 the gape is very considerable, though still very much less than 

 in the adult. 



The opercular flaps overlap ventrally, the left being super- 

 ficial. They still cover the bases of the pectoral fins. The 

 latter are described by Agassiz as being " kept in constant rapid 

 motion, so that the fleshy edge is invisible, and the vibration 

 seems almost involuntary, producing a constant current round 

 the opening leading into the cavity of the gills." 

 The pelvic fins are somewhat more prominent. 

 The yolk-sack, as pointed out by Agassiz, has now dis- 

 appeared as an external appendage. 



After the stage last described the young Fish rapidly ap- 

 proaches the adult form. To shew the changes effected we 

 have figured the head of a larva of about a. month old and 

 23 millims. in length (Plate 34, fig. 15). The suctorial disc, 

 though much reduced, is still prominent at the end of the snout. 

 Eventually, as shewn by Agassiz, it forms the fleshy globular 

 termination of the upper jaw. 



The most notable feature in which the larva now differs in 

 its external form from the adult is in the presence of an ex- 

 ternally heterocercal tail, caused by the persistence of the primi- 

 tive caudal fin as an elongated filament projecting beyond the 

 permanent caudal (Plate 41, fig. 68). 



Delicate dermal fin-rays are now conspicuous in the peri- 

 pheral parts of all the permanent fins. These rays closely 



