LOWER CHUIWATES 



311 



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

 been applied to such animals. 



Under the tunic of one of these ascidians and surrounding the body- 

 proper is an atrial cavity (Fig. 205). 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 

 slits, respiration occurs and food is strained out. On the side of the 

 pharynx which corresponds to the ventral surface is a ciliated groove 

 called the endostyle. A sticky mucous secretion produced in this endo- 



Ducf of hypophys/5 

 Sub^ieura/ g/ar?c/ 

 Afn'a/ fun tie/ 



Anas 



AfnaJ cav/fy 



Dud of ^onaa/ 

 Fsopha^fus 



Ora/ funnel 



Ve/u/-n 



'■r/'a/ ccfvi'fy 

 A/fcyn-f/e 

 Pharynx 



fna/o5-f-y/e 



-Heart 

 In/estine 

 Gonap/ 



Sfomcfch 



Fig. 205. — Anatomy of a typical ascidian. (From Newman, " Vertebrate Zoology," after 



Hertwig.) 



style is continually being passed onward to the intestine and serves to 

 convey food particles to that portion of the alimentary 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 ganglion 

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

 a subneural gland. 



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

 characters. The tunicates are all monecious, and cross-fertilization is the 

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



