STRUCTURE OF THE VERTEBRATES 17 



outpocketing of the stomach-intestine, the hepatic caecum. This 

 blind pouch grows forward on the left side of the pharynx, and 

 assists digestion. From its method of origin it has been identi- 

 fied with the liver of the vertebrates. 



Within the pharynx are the gill slits and their gills. In the 

 larva these are metameric, paired openings; but in the adult 

 metamerism is soon lost, and new gills are added until there 

 may be more than a hundred pairs. The gills serve as respira- 

 tory organs, the water which carries food particles also carrying 

 dissolved oxygen. The course of the water is, therefore, im- 

 portant. It is forced through the mouth into the pharynx. There 

 a portion of its food is dropped, and the oxygenated water is 

 carried out through the gill slits, supplying oxygen to the vascu- 

 lar gills and taking up the carbon dioxide. In the adult an 

 atrium is present, a specialized external covering which protects 

 the gills (page 27). 



Supporting Structures. The main supporting element of 

 Amphioxus is the notochord. This rod of spongy tissue is sur- 

 rounded by two concentric sheaths of connective tissue, the 

 notochordal sheaths, which give strengih to the notochord. 

 Fibers pass out from these sheaths between the muscles, sup- 

 porting them and giving points of attachment to the muscle 

 fibers. Any contraction of the latter exerts a pull on the 

 notochord and affects the entire animal. 



The cartilage of Amphioxus is a very soft tissue having few 

 of the characteristics of true cartilage. These cartilages can be 

 seen in the dorsal fin as the segmental fin rays. A horseshoe- 

 shaped buccal cartilage surrounds the funnel, the open side at 

 the bottom. From this cartilage a small ray extends out into 

 each cirrus. The gill slits are also supported by cartilaginous 

 gill bars which lie between the slits. The illustrations give the 

 details of these structures. 



Muscular System. The muscles are of the simplest nature. 

 The myotomes, or muscle bands, are V-shaped with the apex 

 pointed forward. The muscle fibers run longitudinally, groups 

 of fibers being caught into bundles by connective tissue sheaths, 

 and a large number of bundles forming a myotome. Between 

 each myotome is the myoseptum which is intimately connected 



