Fig. 66. Ventral view of the head of a Lepidosteus embryo shortly before hatching, to shew the large suctorial disc.
m. mouth; op. eye; s.d. suctorial disc.
The general form of Lepidosteus shortly after hatching is shewn in [fig. 67]. On the ventral part of the front of the head is placed the large suctorial disc. At the side of the head are seen the olfactory pit, the eye and the auditory vesicle; while the projecting vesicle of the mid-brain is very prominent above. Behind the mouth follow the visceral arches. The mandibular arch (md) is placed on the hinder border of the mouth, and is separated by a deep groove from the hyoid arch (hy). This groove is connected with the hyomandibular cleft, but I have not determined whether it is now perforated. The posterior border of the hyoid arch is prolonged into an opercular fold. Behind the hyoid arch are seen the true branchial arches.
Fig. 67. Larva of Lepidosteus shortly after hatching. (After Parker.)
ol. olfactory pit; op. optic vesicle; au.v. auditory vesicle; mb. mid-brain; sd. suctorial disc; md. mandibular arch; hy. hyoid arch with operculum; br. branchial arches; an. anus.
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 behind 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 the throat, a well-developed appendage continued backwards beyond the level of the commencement of the intestine. This appendage is no doubt the air-bladder.