366 TEXTBOOK OF ZOOLOGY 



the internal organs through the transparent mantle. Upon viewing 

 this from the left the large saclike pharynx may be seen continuing 

 ventrally and posteriorly from the branchial siphon, finally narrow- 

 ing at its dorsoposterior extremity to become the small tubular esoph- 

 agus which turns sharply downward and anteriorly to become the 

 stomach. The esophagus is partially embedded in a dark-colored 

 digestive gland. The stomach continues anteriorly and upward where 

 it becomes intestine, which turns ventrally on itself in a U-shape. It 

 finally ends with the anus which opens into the atrial cavity shortly 

 below the atrial siphon. A current of water carries food into the 

 digestive system and oxygen for respiratory purposes. The water 

 enters the branchial siphon, passes into the sievelike pharynx, and 

 from here passes through the gill slits or stigmata in its wall into 

 the surrounding atrial cavity, and finally leaves the body by way of 

 the atrial siphon. Oxygen is absorbed by the blood in the walls of 

 the stigmata. The animal's food consists of minute organisms which 

 are entangled in mucus secreted by a glandular groove, the endo- 

 style, which extends from the branchial siphon along the ventral 

 midline of the pharynx to the esophagus. This food mass passes into 

 the esophagus and out through the alimentary canal where digestion 

 and absorption occur. The heart is a contractile tube which pulsates. 

 It lies ventral to the stomach and forces the blood in one direction by 

 a series of contractions and then in the opposite direction by another 

 series. Vessels extend in one direction to the pharynx, primarily, 

 and in the opposite direction to other organs and the body wall. 

 These animals are hermaphroditic or monoecious. Each has two com- 

 pound sets of gonads, one on the left side in the loop of the intestine 

 and the other on the right side of the body. Some of the sessile 

 tunicates, as Molgula, reproduce by budding. There is an oblong, 

 closed excretory sac which may be seen from the right side. The cen- 

 tral nervous system is reduced to a nodulelike ganglion located be- 

 tween the siphons in the dorsal portion. Nerves branch from this to 

 the various parts of the body. The life history of the tunicate is one 

 of interest. Cross- fertilization is the rule; that is, spermatozoa from 

 one individual usually fertilize ova from another ; however, there may 

 be exceptions to this. The fertilization occurs in the water outside 

 the body. The eggs hatch to produce larvae somewhat similar to am- 

 phibian tadpoles which are free-swimming. The larva possesses the 

 typical notochord, gills, and nerve cord of Chordata. For some reason 



