39 



COLLEGE ZOOLOGY 



over three miles. They range in size from about a hundredth 

 of an inch to over a foot in diameter. Some are brilliantly 

 colored. The adult (Fig. 338) is often sac-like and has received 

 the common name " sea-squirt " because when irritated it may 

 eject water through two openings in the unattached end (Fig. 

 338, i, 2). The term tunicata is applied to members of the 



group on account 

 of a cuticular outer 

 covering known as 

 a test or tunic. 



The chordate 

 characteristics of 

 tunicates were not 

 recognized until the 

 development of the 

 egg and metamor- 

 phosis of the larva 

 were fully investi- 

 gated (Kowalevsky, 

 1866). It was then 

 discovered that the 

 typical larva (Fig. 

 339), which is about 

 a quarter of an inch 

 long and resembles 



a frog tadpole pOS- 



sesses (i) a distinct 

 notochord (A, no to), 



(2) a neural tube in the tail which enlarges in the trunk 

 (A, med)j ends in a vesicle (A, sens.ves), and is considered 

 the forerunner of the brain of the VERTEBRATA, and (3) a 

 pharynx which opens to the exterior by ciliated gill-slits 

 (A, stig). The tail propels the larva forward by lateral strokes. 

 After a short existence as a free-swimming organism the larva 

 becomes attached to some object by three projections on the 



FIG. 337. Sketch 01 the chief kinds of TUNICATA 

 found in the sea. (From the. Cambridge Natural 



History.) . 



