80 AMPHIOXUS. 



pharynx at its dorsal end. It does not shift its position in any 

 way ; but, about the stage represented in Fig. 38, when the 

 secondary gill-slits are formed, and the primary slits are moving 

 across to the left side, the club-shaped gland begins to atrophy, 

 and by the stage shown in Fig. 39 has disappeared completely. 

 The function and the morphological meaning of the club- 

 shaped gland are very doubtful. Willey has suggested that it 

 may be the modified first gill-slit of the right side, adducing in 

 support of the suggestion the fact that the first gill-slit of the 

 left side is also a structure which disappears early ; indeed, about 

 the same time as the club-shaped gland itself. It is difiBcult, 

 however, to understand, if the club-shaped gland is formed from a 

 gill-slit of the right side, why its external opening should be on 

 the left side of the head. 



4. The Mouth. 



The most striking features about the mouth, at thfe com- 

 mencement of the larval period, are its position on the left side 

 of the head, and its enormous size. As shown in Fig. 36, the 

 mouth, o, and the first gill-slit, hk i, with the part of the pharyn- 

 geal cavity between them, form a huge opening, perforating the 

 animal from side to side like the eye of a needle. 



During the formation of the primary gill-slits the mouth 

 remains on the left side of the head, and increases considerably 

 in length ; extending, at the close of the stage (Fig. 37, o), from 

 the second to the seventh myotome inclusive. 



From the commencement of the formation of the secondary 

 gill-slits the mouth gradually shifts its position, growing round 

 the anterior end of the pharynx, and eventually attaining the 

 median position and the shape characteristic of the adult. The 

 shifting commences with the formation of a groove on the surface 

 of the head, leading from the preeoral pit to the upper and anterior 

 angle of the mouth. By deepening of this groove the mouth 

 opening becomes placed obliquely across the body, and by a 

 continuance of the process, together with growth forwards of its 

 posterior lip, it ultimately becomes median in position. The 

 mouth is relatively much smaller in the adult than in the larva, 

 but not actually so. 



The margin of the mouth opening of the larva becomes the 

 velum of the adult, from which the velar tentacles arise as out- 



