316 



COLLEGE ZOOLOGY 



Figure 199. Diagram showing some of the fundamental differences in the body plan of {top) 

 an achordate or invertebrate (annelid) and [bottom] a chordate (snake) in the location of the 

 digestive system, heart, and nervous system. The arrows indicate the direction of the flow of blood. 



2. Urochordata (Tunicata). Tunicates (sea 

 squirts) and a number of other marine 

 forms. 



3. Ccphalochordata. Two families of fishlike 

 animals called lancelets. 



4. Vertebrata. Animals with backbones. 



The subphylum Vertebrata includes most 

 of the chorda tes (Fig. 200), but the other 

 three subphyla, often called protochordates, 

 are of considerable interest since they are 

 more primitive and hence give us some idea 

 of the character of the animals from which 

 the vertebrates probably developed. The 

 primitive chordate most frequently studied 

 in general zoology laboratories is the am- 

 phioxus of the subphylum Ccphalochordata. 

 The amphioxus exhibits chordate charac- 

 teristics (notochord, gill slits, and a dorsal 

 tubular nerve cord) so clearly that it illus- 

 trates well the basic chordate characteristics. 

 In the next chapter a detailed description 

 of the frog is presented as an introduction 

 to the subphylum Vertebrata. 



AMPHIOXUS-A 

 PRIMITIVE CHORDATE 



The Ccphalochordata comprise about 30 

 species of marine animals of which Branchi- 

 ostoma lanceolatus, commonly known as the 

 amphioxus, lives in the waters of tropical 

 and temperate seacoasts (Fig. 201). The 

 amphioxus is of special interest, since it 



exhibits the characteristics of the chordates 

 in a simple condition. Furthermore, it may 

 be similar to some ancient ancestor of the 

 vertebrates. 



The amphioxus is about two inches long. 

 The semitransparent body is pointed at both 

 ends and laterallv compressed. It is found 

 near the shore, where it burrows in the clean 

 sand with its head or tail, and conceals all 

 but the anterior end (see headpiece, p. 315). 

 It sometimes leaves its burrow at night and 

 swims about by means of rapid lateral move- 

 ments of the body. When it ceases to move, 

 it falls on its side. 



External anatomy 



The amphioxus (Fig. 201) is shaped like 

 a fish but has no lateral fins and no distinct 

 head. Along the middorsal line is a low 

 dorsal fin which extends the entire length 

 of the body and widens at the posterior end 

 into a caudal fin. The caudal fin extends 

 forward on the ventral surface to form the 

 short ventral fin. Both dorsal and ventral 

 fins are strengthened by rods of connective 

 tissue called fin rays. In front of the ven- 

 tral fin, the lower surface of the body is 

 flattened, and on each side is an expansion 

 of the integument called the metapleural 

 fold (Fig. 202). 



The body wall is divided into V-shaped 

 muscle segments, the myotomes; these are 

 separated from one another by septa of 



