500 



UROCHORDA 



body is invested by a thickeiied cuticular test, which contains 

 cellulose. The relatively large pharynx is perforated by two 

 {in Larvacea), or {in the majority) by numerous ciliated gill- 

 slits, and is surrounded to a greater or less extent by a peri- 

 branchial chamber, which communicates with the exterior 

 by a special dorsal {atrial) opening. The ventral heart is 

 simple and tubidar, and there is a periodic reversal in the 

 direction of the blood current. Nephridia are absent, and the 

 renal organs have no ducts. All are hermaphrodite. There 

 is usually a metamorphosis iti development. Colonies are 

 frequently formed. 



Type of TuNiCATA — a simple Ascidian {Ascidia mentula) 



An adult Ascidia is an irregular oval of 3 to 4 in. in 

 length ; one end is attached to stones or weed ; the other, 

 more tapering, bears the 8-lobed mouth ; close beside this, 

 on the morphologically dorsal surface, lies the 6-lobed ex- 

 halant or atrial aperture. During life, water is constantly 

 being drawn in by the mouth and passed out by the atrial 

 opening. If irritated, the animal may drive a jet of water 

 with considerable force from both apertures, whence the 

 name " sea-squirt." 



Test. — The whole body is clothed in a thick test, some- 

 times called a tunic, though this name is more frequently 

 applied to the underlying body wall. From this body wall 

 the test can be readily removed, the two being unattached 

 except at one spot, where blood vessels pass into the test, 

 and also to a less degree at the two openings. To begin 

 with, this test is a true cuticle, produced by secretory 

 prolongations of the ectoderm cells ; but soon after its 

 formation mesenchyme cells migrate into it, and give rise 

 to patches of connective tissue cells. These cells apparently 

 retain throughout life some phagocytic importance. In 

 Ascidia outgrowths of the body wall with prolongations of 

 blood channels enter the test, ramifying in all directions. 

 In some Ascidians this is carried further, so that the test 

 becomes an important accessory organ of respiration. The 

 test consists in great part of a carbohydrate identical with 

 the cellulose of plants. This " cellulose " or " tunicin " is 

 common throughout the group, but the relative amount 



