ASCIDIA. 425 



test, which contains cellulose. The relatively large pharynx 

 is perforated by two (in Larvaced), or (in the majority} by 

 numerous ciliated gill-slits, and is surrounded to a greater or 

 less extent by a peribranchial chamber, ivhich communicates 

 with the exterior by a special dorsal (atrial) opening. The 

 ventral 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 have no ducts. All are 

 hermaphrodite. There is usually a metamorphosis in 

 development. Colonies are frequently formed. 



Type of TUN ic AT A a simple Ascidian (Ascidia mentiild]. 



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 origin 

 the test is a true cuticle, produced by secretory prolonga- 

 tions 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 

 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 carbo- 

 hydrate 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. 



Body -wall and muscular system.- -The body - wall, 



