ASCIDIA. 397 



cellulose. The pharynx is perforated by two (Larvacea), or 

 {in the majority) by numerous ciliated gill-slits, and is sur- 

 rounded to a greater or less extent by a peribranchial chamber, 

 which communicates with the exterior by a special (atrial) 

 opening. The heart is simple and tubular, and there is 

 a periodic reversal in the direction of the blood current. 

 Nephridia are absent, and the renal organs are devoid of 

 ducts. All are hermaphrodite. There is usually a meta- 

 morphosis in development. Colonies are frequently formed. 



I Type of Tunicata — a simple Ascidian (Ascidia mentula). 



In form an adult Ascidia is an irregular oval of 3 to 

 4 in. in length ; one end is attached to stones or weed ; 

 the other is more tapering, and bears the mouth, close 

 beside which, on the morphological dorsal surface, lies 

 the exhalant or atrial aperture. During life, water is con- 

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

 atrial opening. If irritated, the animal frequently drives 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. In the 

 natural condition of parts, however, the two structures are 

 closely apposed. In origin the test is a true cuticle, pro- 

 duced 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 im- 

 portance. In Ascidia blood vessels also enter the test from 

 the body, and ramify in all directions. In some Ascidians 

 this is carried further, until it may become an important 

 accessory organ of respiration. The test itself consists in 

 great part of a carbohydrate apparently identical with 

 the cellulose of plants; this "cellulose" or "tunicin" is 

 common throughout the group, but the relative amount 

 produced varies markedly in the different forms. 



