PHYLUM CHORDATA 



45 



(ir less than two iuclios. Its form will bo obvious from Fig. 741 

 and from the transverse sections, Fig. 742, A and L'. The'' body 

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

 anterior two-thirds is roughly triangidar in transverse section, 

 presenting right and left sides, incUned towards one another 

 above, and a convex ventral surhxce. 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 fold, the doral Jin (dors./.) : this is continued round 

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

 fin {rent./.}, 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 



nc/i 



cdf 



j^on 



Flu. 741 .— Amphioxus lanceolatus. A, ventral, B, side view of the eiitiic aniinnl. 

 (tn. anus ; atrp. atriopore ; oj.f. caudal fin ; clr. cirri ; dors.f. dorsal fin ; tfor^. f. r. dorsal fin- 



rays ; r/oit. gonads ; 

 vent. f. ventral f 



dpi. mctapleure ; ni>/Oia. myomeres ; ncli. notocliord ; 

 cat. f. r. ventral lin-i-ays. fAfter Kirkaldy.) 



or. hi. oral hood ; 



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 metaphure {intpL), which extends forwards to the oral 

 hood mentioned in the next paragraph. 



Below the pointed anterior extremity is a large median ajjerture 

 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 

 loheel-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 



