304 



GENERAL ZOOLOGY 



lack a backbone have been admitted to resemble the 

 vertebrates in other important respects. These peculiar 

 forms show so many points of agreement with the verte- 

 brates that zoologists now uniformly agree that they should 

 be united with them in a larger group to which the name 

 : 'chordate" is applied. This name is taken from the fact 

 that a soft, rod-like structure called the notochord occurs in 

 the position which is occupied by the backbone of a verte- 

 brate. There are three of these groups of low chordates 

 (Figs. 156, 157, 158) represented by the fish-like lancelet, 

 the sac-shaped tunicate, and the worm-like Balanoglos'sus. 



Nerve cord 



Notochord 



Atrium 



Fig. 156. The lancelet. (x2) 

 Modified from Kowalevsky 



The Lancelet. The lancelet (Amphiox'us lanceola'tus, 

 Fig. 156) is a fish-like animal about two inches long. It 

 lives almost embedded in the sand at the sea bottom in 

 Chesapeake Bay and in other warm ocean waters. 



The mouth of the lancelet opens into a long pharynx, 

 which has many pairs of gill slits. When water and food 

 pass in at the mouth, the water passes through the gill slits, 

 giving up oxygen to the blood in the gills, and then passes 

 into a chamber partially surrounding the pharynx, called 

 the atrium, and to the outside by the atrial pore ; the food 

 goes down the intestine. 



Immediately above the intestine is the structure which 

 corresponds in position to the backbone of higher animals ; 

 it is called the notochord. The notochord is soft throughout 

 life, but it is sufficiently strong to act as a supporting rod 



