Il8 LEPIDOSTEUS. 



There is still a continuous dorso-ventral fin, in which there 

 are as yet no fin-rays, and the anterior paired fins are present. 



The yolk-sack is very large, but its communication with the 

 alimentary canal is confined to a narrow vitelline duct, which 

 opens into the commencement of the intestine immediately 

 beJiind the duct of the liver, which is now a compact gland. The 

 yolk in Lepidosteus thus behaves very differently from that in 

 the Sturgeon. In the first place it forms a special external 

 yolk-sack, instead of an internal dilatation of part of the 

 alimentary tract ; and in the second place it is placed behind 

 instead of in front of the liver. 



I failed to find any trace of a pancreas. There is however, 

 opening on the dorsal side of tJie tJiroat, a well-developed append- 

 age continued backwards beyond the level of the commencement 

 of the intestine. This appendage is no doubt the air-bladder. 



In the course of the further growth of the young Lepidosteus, 

 the yolk-sack is rapidly absorbed, and has all but disappeared 

 after three weeks. A rich development of pigment early takes 

 place; and the pigment is specially deposited on the parts of 

 the embryonic fin which will develop into the permanent fins. 



The notochord in the tail bends slightly upwards, and by the 

 special development of a caudal lobe an externally heterocercal 

 tail like that of Acipenser is established. The ventral paired 

 fins are first visible after about the end of the third week, and by 

 this time the operculum has grown considerably, and the gills 

 have become well developed. 



The most remarkable changes in the later periods are those 

 of the mouth. 



The upper and lower 

 jaws become gradually 

 prolonged, till they event- 

 ually form a snout ; while 

 at the end of the upper ^ 



iaw ij nlared the siirto FlG ' 68 " HEAD OF AN ADVANCED LARVA 



ja\V IS plac OF LEPIDOSTEUS. (After Parker.) 



rial disc, which is now COn- /. openings of the olfactory pit ; sd. remains 



siderably reduced in size of the larval suctorial disc, 

 (fig. 68, sd). The " fleshy globular termination of the upper jaw 

 of the adult Lepidosteus is the remnant of this embryonic 

 sucking disc." (Agassiz, No. 92.) 



