GANOIDEI. 



107 



Before hatching, the embryo has to a small extent become 

 folded off from the yolk both anteriorly and posteriorly ; and has 

 also become to some extent vertically compressed. As a result 

 of these changes, the general form of its body becomes much 

 more like that of an ordinary Teleostean embryo. 



The general features of the larva after hatching are illustrated 

 by figs. 53, 54 and 55. Fig. 53 represents a larva of about 7 mm. 

 and fig. 54 a lateral and fig. 55 a ventral view of the head of a 

 larva of about 1 1 mm. 



There are only a few points which call for special attention in 

 the general form of the body. In the youngest larva figured the 

 ventral part of the hyomandibular cleft is already closed : the 

 dorsal part of the cleft is destined to form the spiracle (sp). The 

 arch behind is the hyoid : on its posterior border is a mem- 

 branous outgrowth, which will develop into the operculum. In 



FIG. 53. LARVA OF ACIPENSER OF 7 MM., SHORTLY AFTER HATCHING. 

 ol. olfactory pit ; op. optic vesicle ; sp. spiracle ; br.c. branchial clefts ; an. anus. 



older larvae, a very rudimentary gill appears to be developed on 

 the front walls of the spiracular cleft (Parker), but I have not 

 succeeded in satisfying myself about its presence ; and rows of 

 gill papillae appear on the hyoid and the true branchial arches 

 (figs. 54 and 55, g). The biserially-arranged gill papillae of the 

 true branchial arches are of considerable length, and are not at 

 first covered by the operculum ; but they do not form elongated 

 thread-like external gills similar to those of the Elasmobranchii. 

 The oral cavity is placed on the ventral side of the head; it 

 has at first a more or less rhomboidal form. It soon however 

 (fig- 55) becomes narrowed to a slit with projecting lips, and 

 eventually becomes converted into the suctorial mouth of the 

 adult. The most remarkable feature connected with the mouth 

 is the development of provisional teeth (fig. 55) on both jaws. 



