146 



THE LANCELET 



Notice the newly developed gill-slits at the extreme end of the pharynx. 



3. The Circulatory System. 



There is no heart," but the circulation of the blood is brought about by the 

 contractile nature of the principal blood-vessels. A dorsal artery, the dorsal 

 aorta, lies under the notochord and extends almost along the whole length of 

 the alimentary canal. In the pharyngeal region it is paired, the two aortas 

 being formed by numerous efferent branchial vessels from the walls of the 

 pharynx. A ventral aorta rvms along the ventral wall of the pharynx. It is 

 contractile and drives the blood forwards to the gill-slits, where the blood is 

 purified. It gives oft a series of afferent branchial vessels on. each side, which 



Dorsal fin 

 Noto.chord / Myotomes » 



_, , ^„ Spinal cord with 



Dorsal ^oru ,^^^.^^ ^^^^^ 



Fin rays 



Cranial nerveS' 



Caudal fin 



Ventral 



Position of Anns' _ 



Subintestinal vein 



aperture ; Atrium ; 

 Intestine Conads Uver 



^Olfactory 

 pit 



„nx \ ', / Oral cirri 



Ventral aorta'', Buccal cartilage 

 Contractile bulb pharyngeal clefts 



Fig. 49. — Amphioxus dissected from the right side. 



pass along the pharynx and communicate with the efferent branchial vessels 

 by means of capillaries. The blood collects in the dorsal aorta and is sent to 

 the posterior region of the body. The aorta supplies the intestine, and the 

 smaller vessels there form the capillaries. The blood is collected from these in 

 a median sub-intestinal vein running beneath the intestine to the liver, where 

 it breaks into a series of capillaries. The blood is collected from the liver by 

 hepatic veins which unite to form the ventral aorta. The bases of the afferent 

 branchials are also contractile and form the contractile bulbs. 



4. The Muscular System. 



The lateral muscles of the body consist of sixty-two segments, the myotomes 

 or myomeres, dovetailed into one another. The myotomes are separated by 

 connective-tissue partitions. The myotomes of the right side alternate with 

 those of the left side. Each is <-shaped and composed of numerous flat 

 striated muscle-plates, arranged longitudinally. 



A set of transverse muscles extends across the ventral surface of the anterior 

 two-thirds of the body ; these are situated in the floor of the atrial chamber, 

 and are the ventral muscles of the body. 



5. The Reproductive System. 



The sexes are separate. The male and female reproductive organs are 

 outwardly similar. The testis (or ovary) consists of twenty-six pairs of 

 cube-shaped sacs lying along the inner wall of the atrium at the lower ends 

 of the tenth to the thirty-fifth myotomes. There are no generative ducts. 



Put the dissected specimen in water in a glass tube, and shake it thoroughly 

 until the spinal cord is separated off. Stain the cord in picrocarmine, and 

 mount it on a slide. 



