PHYLUM CHORDATA 



391 



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

 It then undergoes a retrogressive metamor- , 



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 (Fig. 338) is 

 attached by a stalk (p) and surrounded by a 

 tunic. At the distal end are two openings; 

 one is the mouth (1), 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 (3). A 

 single ganglion, the brain (12), lies between 

 the branchial and atrial tubes. Tunicates 

 are hermaphroditic. The reproductive 

 organs lie near the intestinal loop (8), and 

 their ducts open (4) near the anus. Many 

 species reproduce asexually by budding. 



There are three orders of tunicates (Fig. 

 337): (1) the Ascidiacea, (2) the Thalia cea, 

 and (3) the Larvacea. 



Order 1. Ascidiacea (Fig. 337, lower 

 portion). — The tunicates belonging to this 



Fig. 338. — A Tuni- 

 cate, Ciona inlestinalis 

 1, mouth; 2, atrial ori- 

 fice; 3. anus; 4, geni- 

 tal pore ; 5, muscles ; 

 6, stomach ; 7, intes- 

 tine; 8, reproductive 

 organs; g, stalk; 

 10, tentacular ring ; 

 //, peripharyngeal 

 ring; 12, brain. (From 

 Shipley and Mac- 

 Bride.) 



