STRUCTURE OF 8ALPA. 209 



system, which is situated ventrally between the mouth and 

 vent. We thus have in these Tunicates a front and hind 

 end of the body, a dorsal and ventral, as well as a distinct 

 bilateral symmetry of the body. This is seen in Appendi- 

 cularia as well as in Doliolum and Salpa, however much 

 this symmetry may be obscured in the more typical Ascidi- 

 ans, such as Ascidia, Molgula, Boltenia, etc. 



The oral aperture leading into the respiratory sac is large, 

 being as wide as the body ; the respiratory sac is more com- 

 plicated than in other Ascidians, and more so than in Doli- 

 olum, where it is a wide, deep passage, the oesophagus at the 

 hinder end, the sac itself perforated by two rows of bran- 

 chial slits, four or five slits in each row. In Salpa, how- 

 ever, the respiratory sac, as described by Brooks, is attached 

 to the outer tunic, around the edges of the mouth, as in 

 other Tunicates. There are only two branchial slits, one on 

 each side ; these are very large, and cover almost the whole 

 surface of the branchial sac, except the median dorsal and 

 haemal lines. On the neural side the branchial slit opens 

 directly into the atrium, the ciliated line where the two 

 tunics unite being marked by the so-called "gill" (Brooks). 

 In Salpa, according to Brooks, the branchial sac, though 

 ciliated within, is not so directly concerned in the respiratory 

 act as in other Tunicates, since respiration is effected largely 

 by the action of the muscles, which also assist deglutition, 

 and are the organs of locomotion. These contract rythmi- 

 cally, with great regularity, and at each contraction the 

 water is expelled from the branchial sac through the atrial 

 aperture ; and when the muscles are relaxed, the elasticity 

 of the test distends the chamber, and a fresh supply is drawn 

 in through the branchial aperture, the lips of which readily 

 admit its passage in this direction, while a similar set of 

 valves allows its passage out of the atrial aperture, but pre- 

 vents its return." Thus a chain of individuals move with a 

 uniform motion, while the solitary individuals and those 

 which have been set free by the breaking up of a chain, move 

 by jerks. 



The digestive canal is small, curved on itself, the cesopha- 

 gus leading from the bottom of the pharyngeal or respiratory 



