20 



PHYLUM CEPHALOCHORDA. 



from the development of the parts (see below). The buccal 

 cavity is bounded behind, at the level of the anterior angle 

 of the seventh myotome, by a muscular membrane, the velum, 

 which separates it from the pharynx and is perforated by an 

 aperture, which has been sometimes called the true mouthL 

 and is the actual opening which formed the mouth in the larva. 

 The edges of the velum around this opening bear twelve delicate 

 tentacles, the velar tentacles, which project backwards into 

 the pharynx. The buccal cavity is lined by a ciliated epithelium, 

 and bears on its roof slightly to the right of the median line 

 a pit called Hatschek's pit, or simply the preoral pit (Figs. 8, 9). 

 This organ is lined by a columnar epithelium, the cells of which 

 bear stiff sensory hairs. It opens in a groove of columnar 

 finely ciliated epithelium, which extends a short distance in 

 front of it in the roof of the mouth and behind bifurcates 

 into two ciliated grooves ; these pass obliquely backwards and 

 outwards to the velum, and then pass ventrally along the 

 jimction of the velum and sides of the mouth. They terminate 

 either by simply coming to an end, or by running into one 

 another on the floor of the buccal cavity. These two diverging 



J ciliated furrows 



give off access- 

 ory furrows, 

 which are 

 formed by folds 

 of the antero- 

 lateral walls of 

 the main fur- 

 rows. The ac- 

 cessory ciliated 

 furrows pass for- 

 wards for a 

 short distance 

 on the roof and 

 sides of the 

 mouth. The 

 whole structmre 

 constitutes the 

 " wheel - organ " 

 of J. Miiller. It 



' ^ » 10 n " 



Fig. 8. — View of the roof of tlie buccal cavity of AmpMoxus 

 lanceolatus from below (after v. Wijhe), 1 posterior wall 

 of buccal cavity ; 2 ciliated groove of left side ; 3 left side 

 of preoral hood ; 5 notochord ; 6 second myotome ; 

 7 Hatschek's (preoral) pit ; 8 ciliated pit ; 9 right side of 

 preoral hood ; 10 third myotome ; ll ciliated groove of 

 right side ; 12 accessory ciliated groove ; 13 sphincter 

 muscle of velum : li velar tentacles. 



