THE EVOLUTION OF INVERTEBRATES 



303 



A and Z?), have nothing in common with vertebrates. Sea- 

 squirts live attached to rocks and wharves, and once attached 

 never leave the place. The body 

 is covered with a tough coat or 

 tunic, which gives the class its 

 name, Tunica 1 ta. The food and 

 oxygen are drawn through the 

 opening in the upper tube, and 

 the excess water and wastes are 

 discharged by the lower tube. 

 There is a pharynx Avith gill- 

 openings, and a nervous system, 

 but there is no indication of a 

 notochord. 



The adult tunicate exhibits 

 a lower degree of organization 

 than the larva. The larva (Fig. 

 150) has very much the form 

 of a frog-tadpole, and it swims 

 about. Its locomotor organ is a 

 fin-like tail (Fig. 150, 12). As 

 long as the animal remains a 

 larva it has structures which 

 indicate vertebrate relationship. 

 Extending through the middle 

 of the tail is a notochord (Fig. 

 150, 6), which is evidently a sup- 

 porting organ. Dorsal to the 

 notochord is the nerve-cord (Fig. 

 150, 7). Below the notochord is 

 the alimentary canal (Fig. 150, 3), and near by are the begin- 

 nings of the gills, with openings (Fig. 150, 2) which, however, 

 are not gill-slits. When the larva reaches a certain stage of 

 development, it fastens itself by adhesive papilla? (Fig. 150, 11) 



11 



FIG. 150. Larva of Tunicate. 

 Much enlarged. (After Herd- 

 in an) 



1, incurrent opening ; 2, gill-open- 

 ings ; 3, intestine ; 4, anus ; 5, ex- 

 current opening ; 6, notochord ; 

 7, nerve-cord ; 8, eye ; 9, otocyst ; 

 10, endostyle; 11, adhesive pa- 

 pillae; 12, portion of tail 



