324 MANUAL OF ZOOLOGY sect. 



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

 (vent./), as far as the spot where the oval gives place to the trian- 

 gular transverse section. 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 (cd.f). 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 

 forward 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. Ad), the edge 

 of which is beset with numerous tentacles or cirri. The oral hood 

 encloses a cup-shaped cavity or vestibule, at the bottom of which is the 

 mouth (Fig. 204, mth). Immediately in front of the anterior termina- 

 tion of the ventral fin and partly enclosed by the metapleures is a 

 rounded aperture of considerable size, the atripore (atrp), and a short 

 distance from the posterior extremity of the body is the anus (an), 

 placed unsymmetrically on the left side of the ventral fin. The post- 

 anal portion of the body is distinguished as the tail. 



Amphioxus ordinarily lives with the greater part of the body buried 

 in sand, only the anterior end with the expanded oral hood protruding. 

 It also swims in the vertical position, and frequently lies on one side 

 on the sand; it burrows, head foremost, with great rapidity. It occurs 

 on the American coast as far north as Cape Hatteras. A current of 

 water is constantly passing in at the mouth and out at the atriopore. 



The muscular layer (my) is remarkable for exhibiting metameric 

 segmentation. It consists of a large number — about sixty — of muscle 

 segments or myomeres, separated from one another by partitions of 

 connective tissue, the viyocommas, and having the appearance, in a 

 surface view, of a series of very open V's with their apices directed 

 forwards (Figs. 203 and 204). 



The chief of the skeletal or supporting structures of the lancelet is 

 the notochord (Figs. 203 and 204, nch), a cylindrical rod, pointed at 

 both ends, and extending from the anterior to the posterior end of the 

 body in the median plane. It lies immediately above the enteric tract 

 and between the right and left myomeres. It is composed of a peculiar 

 form of cellular tissue, known as notochordal tissue, formed of large 

 vacuolated cells extending from side to side of the notochord, and 

 having the nuclei confined to its dorsal and ventral regions, Around 



