xii PHYLUM CHORDATA 523 



body and- extends forwards, as the ventral fin (vent.fi), as far as the 

 spot where the oval gives place to the triangular 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 Jin (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 longitudi- 

 nal 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. The oral hood 

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

 M0ttM(Fig. 191, nitk). 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 (atrp), and a short distance 

 from the posterior extremity of the body is the amis (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 pro- 

 truding. It also swims in the vertical position, and frequently lies on 

 one side on the sand : it burrows, head foremost, with great rapidity. 

 A current of water is constantly passing in at the mouth and out at 

 the atriopore. 



The muscular layer is remarkable for exhibiting metameric seg- 

 mentation. It consists of a large number about sixty of muscle seg- 

 ments or myomeres (myotn.), separated from one another by partitions of 

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

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

 forwards. 



The chief of the skeletal or supporting structures of the Lancelet, is 

 the no foe hard (Figs. 190 and 191, nek.), 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 



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