LARVAL DEVELOPMENT. 145 



connective tissue lying below the epidermis, and repre- 

 sents the rudiment of the first element of the cartilagi- 

 nous skeleton of the buccal cirri. 



Running parallel with the lower margin of the mouth, 

 and curving gently upwards to the dorsal wall of the 

 pharynx, is a ciliated band proceeding from the lower limb 

 of the endostyle, and corresponding to the one on the other 

 side, which we found in connexion with the upper portion 

 of the endostyle. Its course on the left side is somewhat 

 different anteriorly from that of the right side, owing to 

 the position and size of the mouth. (Cf. Figs. 78 and 81.) 



The so-called olfactory pit, which arose at a much earlier 

 stage as an ectodermic depression above and in connexion 

 with the neuropore, no longer lies in the mid-dorsal line as 

 in Fig. 64, but it has been displaced to the left side by the 

 upgrowth of the dorsal fin (Fig. 81). Here, as in the case 

 of the anus, the development of a median fin has no other 

 effect on the aperture in question than to cause it to 

 forsake its primitively median and symmetrical position 

 and to assume an asymmetrical position on the left side of 

 the body. This is important to bear in mind, as the asym- 

 metrical position of the mouth will be explained below on 

 an analogous basis. 



For the present it is sufficient to call attention to the 

 fact that, with the exception of the gill-slits, whose primary 

 unpaired character is due to the retarded or latent develop- 

 ment of their antimeres, the unpaired openings in the 

 body-wall namely, neuropore, praeoral pit, external aper- 

 ture of club-shaped gland, mouth, and anus all lie on the 

 left side of the body. 



At a slightly later stage than the preceding, the front 

 end of the mouth is found to be no longer pointed, but to 

 have become rounded off, and, moreover, to lie at a deeper 



