II. TUNICATA: THALIACEA 



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Order III. Thaliacea (Salpaeformes). 



These, like the Lucias and Copelate, are pelagic, and play an important par- 

 in the plankton. In form a Salpa may be compared to a barrel consisting extert 



A B 



Fig. 503. — .4, B, Salpa democraika with stolon, ventral and lateral views; C, Salpa 

 mucronala, part of a young chain not yet separated, a, anus; c, tunic; d, digestive tract; 

 e, atrial opening; en, endostyle;/, peripharyngeal groove; g, ganglion with horseshoe- 

 shaped eye, and near it the tentacle and hypophysial groove; h, testis; i, mouth; k, gill; 

 m, muscle hoops; st, stolo prolifer. 



nally of a cellulose tunic, lined internally with six or eight circular muscles, not 

 always closed rings, like hoops. By their contraction 

 the water is expelled through the posterior or atrial end 

 of the body, while on their relaxation fresh water enters 

 the other or oral aperture. By the reaction the ani- 

 mals swim through the water with the oral end for- 

 ward. The cavity of the barrel corresponds to pharyn- 

 geal and peribranchial chambers of the ascidian. In 

 the Dolioliid;e the two chambers are separated by a 

 partition perforated by gill slits (fig. 504); in Salpa the 

 partition is reduced to a bar with transverse rows of 

 cilia so that branchial and peribranchial chambers are 

 not distinct; yet the endostyle and the peripharyngeal 

 band are retained. 



The viscera lie in the muscular sac, where the bran- 

 chial bar and the endostyle meet and are usually com- 

 pacted into a mass, the 'nucleus' (intestine, liver, gon- 

 ads, heart). The ganglion is distinct 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 olSalpce have been known, one solitary, the other 

 consisting of numerous individuals connected together like a chain or a rosette 



Fig. 504. — Doliolum 

 denliculalum. (For e.x- 

 planation of letters see 

 fig- 503-) 



