PHYLUM CHORDATA 



391 



anterior end (Fig. 339, A, adti) which secrete a sticky fluid. 

 It then undergoes a retrogressive metamor- t 



phosis during which the tail with the noto- 

 chord and neural tube disappear, and other 

 changes take place as shown in Figure 339. 



The typical adult tunicate (Fi^. 338) is 

 attached by a stalk (p) and surrounded by a 

 tunic. At the distal end are two openings; 

 one is the mouth (i), or branchial aperture, 

 into which a current of water passes; the 

 other (2) is the atrial orifice through which 

 the water escapes to the outside. This 

 current of water brings food into the ali- 

 mentary canal, furnishes oxygen for respira- 

 tion, and carries away excretory substances. 

 Near the mouth is a ring of tentacles (10) 

 forming a sensory sieve through which in- 

 coming water and food must pass. Micro- 

 scopic plants and animals are entangled in 

 mucus secreted by a pharyngeal groove or 

 endostyle (Fig. 339, C, end} which forms a 

 peripharyngeal band (Fig. 338, 11). The 

 alimentary canal is bent upon itself (6, 7), 

 and opens into the atrial cavity (j). A 

 single ganglion, the brain (12), lies between 

 the branchial and atrial tubes. Tunica tes 

 are hermaphroditic. The reproductive FIG. 338. ATuni- 



v ,r , , i i / \ i cate, Ciona intestinalis . 



organs he near the intestinal loop (8), and 7> m ' outh; 2f atri alori- 

 their ducts open (4) near the anus. Many fice; 3. anus; 4, geni- 

 species reproduce asexually by budding. 



There are three orders of tunicates (Fig. tine; 8, reproductive 

 337) : (i) the ASCIDIACEA, (2) the THALIACEA, 

 and (3) the LARVACEA. 



Order i. Ascidiacea (Fig. 337, lower 



portion). The tunicates belonging to this Bride.) 



ii, peripharyngeal 

 ring; 72, brain. (From 

 Shipley and Mac- 



