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COLLEGE ZOOLOGY 



Notochord 

 ■Nerve cord 

 Fin ray 



Myotome 

 Dorsal fin 



Velar tentacle 



Mouth 



Oral tentacle 



"Liver" 

 Gill slit of pharynx 

 ^Gill bar of pharynx 



Caudal fin 

 Anus 



' — Ventral fin 



Figure 201. Amphioxus, an animal that illustrates the three fundamental chordate charac- 

 teristics. An adult with part of body wall removed from the left side to show the general struc- 

 ture. Natural size about two inches long. 



Internal anatomy 

 and physiology 



Skeleton 



The amphioxus has a well-developed 

 notochord (Figs. 201 and 202), which is 

 the main support of the body. This is a 

 rod of connective tissue lying near the 

 dorsal surface and extending almost the 

 entire length of the body. The notochord 

 is composed of vacuolated cells which are 

 made turgid by their fluid contents, and are 

 therefore rigid. Other skeletal structures are 

 the connective tissue rods which form the 

 fin rays, and similar structures that support 

 the oral tentacles (cirri) of the oral hood 

 and gill bars. 



Digestive system 



The food of the amphioxus consists of 

 minute organisms which are carried into the 

 mouth with the current of water produced 

 by cilia on the gills. The mouth is an open- 

 ing in a membrane, the velum, and may be 

 closed by circular muscle fibers which sur- 

 round it. Twelve sensory-oral or velar 

 tentacles protect the mouth, and when 

 folded across it, act as a strainer, thus pre- 

 venting entrance of coarse solid objects. 

 The funnel-shaped vestibule is the cavity 



of the oral hood. The 22 ciliated tentacles 

 which project from the edge of the oral 

 hood are provided with sensory cells. The 

 inner wall of the oral hood bears a number 

 of ciliated lobes and is known as the wheel 

 organ because, during life, its cilia appear 

 to produce a rotatory movement. Water is 

 drawn into the mouth chiefly by the action 

 of the gill cilia. 



The mouth opens into a large, laterally 

 compressed pharynx. A ciliated middorsal 

 groove in the pharynx is called the hyper- 

 branchial groove. A ventral groove, the 

 hypobranchial groove (endostyle), is also 

 present. The endostyle consists of a median 

 ciliated region with a glandular portion on 

 both sides. The glands secrete strings of 

 mucus in which food particles become en- 

 tangled as the mucus passes up the gills. 

 The cilia drive this mucus forward and up- 

 ward, by way of the gills and two peri- 

 pharyngeal grooves, into the hyperbranchial 

 groove. From here it is carried by the hyper- 

 branchial cilia into the intestine. Other 

 food particles are caught by mucus produced 

 in the hyperbranchial groove and then car- 

 ried posteriorly by the action of cilia to the 

 intestine. A ventral finger-shaped outpocket- 

 ing of the intestine is known as the "liver," 

 but it might better be called the midgut 

 cecum, since it is an outgrowth of the mid- 



