UROCHORDA OR TUNICATA 



193 



on one side, is the atrial aperture, for the exit of the current. 

 Sea-squirts are destitute of head and limbs. The ventral heart 

 enclosed in a pericardium is situated between the gill region and 

 the stomach. This heart has the peculiarity of changing the 

 direction of its contractions. When the blood has been driven 

 to the gills for a while it rests a little, and then forces the blood 

 in the opposite direction. 



nn a 



Fig. 155. — Diagram of the growth of a sea-squirt or ascidian: A, a, young 

 free-swimming stage; a', intermediate stage just before becoming fixed. 

 B, b, Full-grown sea-squirt, rooted to the sea bottom and incapable of 

 movement: m, mouth; e, hollow brain with eye; g, gill-slits; h, heart; r, rod 

 of gristle in free-swimming form; nv, nerve cord in same; t, tail in process 

 of absorption in intermediate form. (After Haddon.) (From Baskett, 

 " The Story of the Fishes," D. Appleton and Co., Publishers.) 



The "sea-squirts" were formerly called "Tunicates," until a 

 study of their larval stage showed them to have vertebrate char- 

 acteristics. The larva has a slender finned tail containing a 

 notochord and a nerve cord. They furnish an example of retro- 

 grade development. They are free for a few hours, then be- 

 come fixed and lose the notochord and nearly all traces of their 

 vertebrate characteristics which promised a higher develop- 

 ment. 



