io8 



ACIPENSER. 



These teeth were first discovered by Knock (No. 88). They do not 

 appear to be calcified, and might be supposed to be of the same nature as 

 the horny teeth of the Lamprey. They are however developed like true 

 teeth, as a deposit between a papilla of subepidermic tissue and an 

 epidermic cap. The substance of which they are formed corresponds 

 morphologically to the enamel of ordinary teeth. As they grow they pierce 

 the epidermis, and form hollow spine-like structures with a central axis 

 filled with subepidermic (mesoblastic) cells. They disappear after the third 

 month of larval life. 



In front of the mouth two pairs of papillae grow out, which 

 appear to be of the same 

 nature as the papillae on 

 the suctorial disc in the 

 embryo of Lepidosteus 

 (vide p. 115). They are 

 very short in the embryo 

 represented in fig. 53; 

 soon however they grow 

 in length (figs. 54 and 

 55, st} ; and it is pro- 

 bable that they become 

 the barbels, since these occupy a precisely similar position 1 . 



The openings of the nasal pits are at first single ; but the 

 opening of each becomes 

 gradually divided into 

 two by the growth of a 

 flap on the outer side 

 (fig. 54, ol}. It is prob- 

 able that this flap is 

 equivalent to the fold of 

 the superior maxillary 

 process of the Amniota, 

 which by its growth roofs 

 over the open groove 

 which originally leads from the external to the internal nares ; 

 so that the two openings of each nasal sack, so established in 

 these and in other fishes, correspond to the external and 

 internal nares of higher Vertebrata. 



ol 



ft ' *** 



g p 



FIG. 54. SIDE VIEW OF A LARVA OF ACIPEN- 

 SER OF II MILLIMETRES. 



op. eye ; ol. olfactory pit ; st. suctorial (?) pro- 

 cesses ; m. mouth ; sp. spiracle ; g. gills. 





FIG. 55. VENTRAL VIEW OF A LARVA OF 

 ACIPENSER OF 11 MILLIMETRES. 



;. mouth ; st. suctorial (?) processes ; op. eye ; 

 g- gills. 



1 If these identifications are correct the barbels of fishes must be phylogenetically 

 derived from the papilla? of a suctorial disc adjoining the mouth. 



