508 CHORDATA. 



Order II. Tethyoidea (Ascidiaeformes). 



With the exception of the pelagic Pyrosomidae all of the ascidi- 

 ans are attached to rocks, etc., in the sea. The greater necessity 

 for protection caused by this sedentary life has resulted in a great 

 development of the cellulose tunic, which, enveloping the internal 

 organs, gives these animals a swollen, somewhat shapeless appear- 

 ance. Two openings, mouth and atrial opening, lead into the 

 interior, and the water which issues from these, when the animals 

 are taken from the ocean, has given them the common name of 

 * sea-squirts/ 



On removing the tunic, which is but slightly attached to the 

 other parts except at mouth and atrial opening, a muscular sac is 

 seen (fig. 545), the fibres running circularly and longitudinally. In- 

 side this sac are the viscera, the pharyngeal region by far the most 

 conspicuous. The mouth leads to a short tube with tentacles (/), 

 and then to the pharynx, a wide sac which 

 lies in a large cavity, the peribranchial 



fl ^^f^WW^ "~\ c h amDer > the walls of the pharynx and 

 ! ' vliiff] M n fl ) ^^ ^ e enclosing space uniting on the ventral 

 U liMJlMMJI I side (fig. 543). The pharyngeal walls are 

 perforated like a net by small ciliated 

 gill slits, arranged in longitudinal and 

 transverse rows (fig. 546), through which 

 the water received from the mouth passes 

 into the peribranchial chamber and thence 



FlG. 546. dona intestinalis, a 



bit of the wall of the gill sac to tiie atrium, and so out to the external 



enlarged to show the gill , , 



slits. world. 



While the respiratory water thus passes out in a nearly direct 

 course, the food particles which it contains pass into the digestive 

 tract. By means of a ciliated tract (peripharyngeal band) just 

 inside of the tentacles and surrounded by mucus secreted by the 

 endostyle (or hypobranchial groove), the food is carried back to 

 the oesophagus (oe) at the base of the gill chamber, and thence to 

 the stomach (usually provided with liver glands), and on to the 

 intestine. The anus is at the base of the special portion of the 

 peribranchial chamber, which also receives the genital ducts and 

 hence is known as the cloaca or atrium. 



In the body cavity, which is greatly reduced in the species 

 with concentrated bodies, occur the digestive tract, the sexual 

 organs, and the heart; the latter, frequently S-shaped, extends be- 

 tween the stomach and the endostyle. Opposite to the endostyle 



