//. TUNICATA: THALIACEA. 



511 



closed rings, like hoops. By their contraction the water is expelled 

 through the posterior or atrial end of the body, while fresh water 

 on their relaxation enters the other or oral aperture. By the 

 reaction the animals swim through the water with the oral end in 

 front. The cavity of the barrel corresponds to pharyngeal and 

 peribranchial chambers of the ascidian. In the Dolioliidae the two 



FIG. 550. A, B, Salpa democratica with stolon, ventral and lateral views; C, Salpa 

 mucronata, part of a young chain not yet separated, a, anus; c, tunic; d, diges- 

 tive tract; e, atrial opening; en, endostyle; /, peripharyngeal groove; g, gan- 

 glion with horseshoe-shaped eye, and near it the tentacle and hypophysial 

 groove; /t, testis; t, mouth; /c, gill; m, muscle hoops; st, stolo prolifer. 



chambers are separated by a partition perforated by gill slits (fig. 

 551) ; in the common Salpce the partition is reduced to a bar with 

 transverse rows of cilia so that branchial and peribranchial cham- 

 bers are not distinct; yet the endostyle and the peripharyngeal 

 band are retained. 



The viscera lie in the muscular sac, where the branchial bar and 

 the endostyle meet and are usually compacted into a mass, the 

 i nucleus ' (intestine, liver, gonads, heart). The ganglion is dis- 

 tinct and lies dorsally opposite the endostyle, just in front of the 

 branchial bar. Associated with it is a horseshoe-shaped eye. 



For a long time two kinds of Salpce have been known, one 

 solitary, the other consisting of numerous individuals connected 

 together like a chain or a rosette (fig. 550, C). At the beginning 

 of the last century the poet Chamisso discovered that the chain 



