LOWER CHORD AT ES 



317 



able force. This reaction suggests another name, sea squirts, that has 

 been applied to such animals. 



Inside the body wall of one of these ascidians and surrounding the 

 large pharynx proper is an atrial cavity (Fig. 218). The body always has 

 two openings, one known as the oral funnel, which is the mouth opening, 

 and the other as the atrial funnel, or atriopore, which opens into the atrial 

 cavity. Water enters the oral funnel, passes into a pharynx in the walls 

 of which are numerous pharyngeal slits, through these into the atrial 

 cavity, and out through the atriopore. As the water passes through 

 the pharyngeal sHts, respiration occurs and food is strained out. On 

 the side of the pharynx which corresponds to the ventral surface is a 

 cihated groove called the endostyle. A sticky mucous secretion produced 



£sopha^us ■ 



•~Sfom<^ch 



Fig. 218. — Anatomy of a typical ascidian. {From Newman, " Vertebrate Zoology," after 



Hertwig.) 



in this endostyle is continually being passed onward to the intestine and 

 serves to convey food particles to that portion of the ahmentary canal, 

 where digestion and absorption take place. The anus is situated near the 

 atriopore and the water which passes out through the atriopore carries 

 with it the feces. The heart is a pulsating tube, lying ventral to the 

 stomach, which drives the blood first one way and then the other by 

 alternate series of beats opposite in effect. The adult animal possesses 

 one of the characteristics of a chordate in having pharyngeal slits, but it 

 has no notochord and the nervous system consists only of a ganghon 

 embedded in the body wall between the two funnels and associated with 

 a subneural gland or hypophysis. 



A study of the development of a tunicate reveals all of the chordate 

 characters. The tunicates are all monecious, but cross-fertiUzation is the 

 rule, fertilization taking place outside the body. From the egg is pro- 



