44 



ZOOLOGY 



SECT. 



the posterior end of the body and extends forwards, as the ventral 

 fin (vent. /.), as far as the region where the oval transverse section 

 passes into the triangular. The portion of the continuous 

 median fold which extends round the pointed posterior extremity 

 of the body is somewhat wider than the rest, and may be 

 distinguished as the caudal fin (cl. /.). In the anterior two-thirds 

 of the body there is no median ventral fin, but at the junction 

 of each lateral with the ventral surface is a paired longitudinal 

 fold, the metapleure (mtpl.), which extends forwards to the oral 

 hood mentioned in the next paragraph. 



Below the pointed anterior extremity is a large median aperture 

 surrounded by a frill-like membrane, the oral hood (or. hd.), the 

 A B 



nch 



myorn. 



coe,l 



FIG. 751. Amphioxus lanceolatus. A, transverse section of the pharyngeal region. 

 a. dorsal aorta ; b. atrium ; c. notochord ; co. ccelome ; e. endostyle ; g. gonad ; kb. branchial 

 It lamellae ; kd. pharynx ; 1. liver ; my. myomere ; n. nephridhim ; r. neuron ; sn. spinal 

 nerves. JB, transverse section of the intestinal region, air. atrium ; cod. ccelome ; 

 d. ao. dorsal aorta ; int. intestine ; myom. myomere ; nch. notochord ; neu. neuron ; s. int. v. 

 sub-intestinal vein. (A, from Hertwig. after Lankester and Boveri : B, partly after 

 Rolph.) 



edge of which is beset with numerous tentacles or cirri (dr.). The 

 oral hood encloses a cup-shaped cavity or vestibule, at the bottom 

 of which is the mouth (Fig. 752, mth.). On the wall of the oral 

 hood is a specially modified tract of the epithelium divided into 

 finger-shaped lobes. The cells of this tract, which is known as 

 the wheel-organ, are provided with long cilia, the movements of 

 which drive currents of water with floating food-particles backwards 

 into the pharynx. Along the roof of the vestibule runs a ciliated 

 groove the groove of Hatschek. Immediately in front of the 

 anterior termination of the ventral fin and partly enclosed by the 

 metapleures is a rounded aperture of considerable size, the atriopore 



