XIII 



PHYLUM CIIORDATA 



45 



or less than t\v<j inches. Its iunn will hv obvious from Kig. 741 

 and i'vom the transverse sections, Fig. 742, A and B. The body 

 is elongated, pointed at eitlier end, and compressed. The 

 anterior two-thirds is roughly triangulai- in transverse section, 

 presenting right and left sides, inclined towards one another 

 above, and a convex ventral surface. The posterior third is 

 nearly oval in section, the right and left sides meeting above and 

 below in a somewhat sharp edge. 



Extending along the whole of the dorsal border is a median 

 longitudinal i'old, the dor al fin (dors, f.) : this is continued round 

 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 



or.hcL 



B myom (forsyr- 



dors^ 



jTVi/om 



n<:h 



Fi(i. 741.— Amphioxus lanceolatus. A, ventral, B, side view of the entire animal, 

 (in. anus ; atifi. atriopoi'L- ; fi.f. caudal fin ; cir. cirri; dors./, dorsal fin ; dom. /. r. dorsal fin- 

 rays ; (jon. gonads ; mtpl. nietapleure ; mi/ont. myomeres ; nek. notocliord ; or. ltd. oral hood ; 

 vent. f. ventral fin ; vent. f. r. ventral fin-rays. (After Kirkaldy.) 



distinguished as the caudal fin {d. /.). 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 metaijleure {mt])l.), 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 

 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. 743, 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. Immediately in front of the anterior termination of 

 the ventral fin and partly enclosed by the metapleures is a rounded 



