Classification of Chordates: Protochordata 



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TENTACLES 



ENDOSTYLE 



PERIBRANCHIAL 



NERVE CORD 



ASCIDIA - A UROCHORDATE 



Fig. 309. Ascidia, a urochordate. The ani- 

 mal is viewed as if cut in sagittal section and 

 seen from the right side. (Redrawn from 

 Sewertzolf, after Boas. Courtesy, Neal and 

 Rand: "Chordate \natomy," Philadelphia, The 

 Blakiston Company.) 



Of the two external apertures, the more ventral is the inhalant or 

 oral "siphon" and the other the exhalant or atrial "siphon."* The 

 former leads directly to the mouth, which is surrounded by a circle of 

 tentacles. The mouth opens into a greatly enlarged pharynx, which is 

 perforated by numerous gill-slits or stigmas. The action of the cilia on 

 the bars between these slits serves to maintain a current of water from 

 the pharynx into the surrounding peribranchial (atrial) cavity. Along 

 the floor of the pharynx extends a longitudinal grooved thicken- 

 ing, the endostyle, whose surface is ciliated and coated with secreted 

 mucus. A similar groove extends along the dorsal wall of the pharynx, 

 terminating posteriorly at the opening of the esophagus. A circular 

 peripharyngeal ciliated groove joins the anterior ends of the endostyle 

 and the dorsal groove. Particles of food in the water are caught in 

 the mucus and transported along these grooves into the esophagus 

 by ciliary action. Posterior to the pharynx, the alimentary canal 

 consists of a short esophagus, a spherical stomach, and an intestine 

 which leads to an anus opening into the atrial chamber. 



The heart lies ventral to the esophagus in a coelomic (pericardial) 

 chamber. There are no closed blood-vessels, but the blood is pumped 

 from the heart forward to the pharynx through irregular spaces 

 (lacunas) which resemble functionally the afferent brachial vessels 



