564 The Animal Kingdom 



hesive glands. After a few hours, this free-swimming larva attaches it- 

 self to a substrate by means of the adhesive glands ; thus it lies vertical in 

 position. Metamorphosis occurs rather rapidly and the strictly chordate 

 features disappear as the tail is absorbed. 



THE SUBPHYLUM CEPHALOCHORDATA 



The members of this relatively homogeneous subphylum show their 

 affinities to the true vertebrates much more clearly than do the tunicates. 

 Chiefly this is due to the fact that the notochord is retained throughout 

 life, and the dorsal tubular nervous system persists. These small, fish- 

 like animals (about 2^ inches long) are found in large numbers in sandy 

 bottoms below low tide mark. They are often dredged up in great num- 

 bers from waters of 30 to 40 feet in depth. 



The several genera of this subphylum are all grouped in but one 

 class, Leptocardii. The name amphioxus has been applied in gen- 

 eral to all cephalochordates ; the genus Branchiostoma is abundant in our 

 coastal waters. 



The Anatomy of Branchiostoma. — These small, pinkish animals 

 have an elongate body which is slender and compressed. It is pointed at 

 both ends and without distinct head development. Dorsally there is a 

 fin which extends the length of the body ; posteriorly there is a ventral 

 fin which meets the dorsal. Both then enlarge to form the tail fin. All 

 these fins are strengthened by fin rays. Anterior to the ventral fin on 

 each side of the body is a metapleural fold. In this region the body is 

 flattened ventrally. The mouth is at the ventral portion of the anterior 

 end of the body, and the anus is at the posterior end. Just anterior to the 

 anus is the atrial pore. 



CIRRUS 

 VELUM, 

 PHARYNX 



,FIN RAY I 



^NERVE CORD / 



MYOTOME 

 TA1U_ 



Fig. 188. — The anatomy of amphioxus, Branchiostoma. 



On the sides of the body, the segmental muscle masses or myo- 

 tomes may be seen through the thin epithelium. Around the mouth 

 is an oral hood which has a ring of cartilage to strengthen it. Along this 



