444 SUB-PHYLUM UROCHORDA OR TUNICATA. 
body is invested by a thickened cuticular test, which contains 
cellulose. The relatively large pharynx 1s perforated by twe 
(tn Larvacea), or (in the majority) by numerous ciliated gill- 
slits, and is surrounded to a greater or less extent by a 
peribranchial chamber, which communicates with the exterior 
by a special dorsal (atrial) opening. The ventral heart ts 
simple and tubular, and there is a pertodic reversal in the 
direction of the blood current. Nephridia are absent, and the 
renal organs have no ducts. All are hermaphrodite. There 
7s usually a metamorphosis in development. Colonies are 
Jrequently formed. 
Type of Tunicata—a simple Ascidian (Ascdta mentula) 
An adult Ascédia 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 morphological 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 
