XIII 



PHYLUM CHORDATA 



39 



Amphioxus is a small transparent animal, occurring near the 

 shore and burrowing in sand : its length does not exceed 5 '8 cm. 

 or less than two inches. Its form will be obvious from Fig. 696 

 and from the transverse sections, Fig. 697, A and B. The body 

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

 anterior two-thirds is roughly triangular 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 fold, the dorsal fin (dors. /.) : this is continued round 

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

 fin (cent. /.), as far as the spot where the oval gives place to the 

 triangular transverse section. The portion of the continuous 



an, 



or.hcL. 



B mj/om Jors.fr 



dorsf 



mj/om 



cir , , 



or.hd 



FIG. 696. Amphioxus lanceolatus. A, ventral, B, side view of the entire animal- 

 em, anus ; atrp. atriopore ; cd. /. caudal fin ; cir. cirri ; dors. /'. dorsal fin ; dors. /. r. dorsal fin 

 rays ; gon. gonads ; rntpl. metapleure ; mt/om. myonieres ; nch. uotochord ; or. hd. oral hood ; 

 vtiit. /. ventral fin ; vent. /. r. ventral fin rays. (After Kirkaldy.) 



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 (cd. /.). 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 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. 698, mth). Immediately in front of the anterior 

 termination of the ventral fin and partly enclosed by the meta- 

 pleures is a rounded aperture of considerable size, the atriopore 

 (atrp), and a short distance from the posterior extremity of the 

 body is the anus (an), placed unsymmetrically on the left side of 



