MUSEUM OF COMPARATIVE ZOOLOGY. 301 



The branchial sac presents all the characteristics already described 

 as peculiar to this structure. It is attached to the outer tunic 

 around the edges of the branchial aperture (e), the lips of which are 

 reflected inward and provided with a complicated system of muscles 

 (Figs. 3, 24, and 33, /') ; the whole forming a valve which usually 

 prevents the water from passing outward, although it may be so 

 arranged, by the contraction of the muscles, that the contents of the 

 branchial sac may be violently expelled through it. 



Owing to the transparency of Salpa, the endostyle is very promi- 

 nent, and has the appearance of a solid rod in the adult, although 

 earlier stages show that it is simply a furrow between two longitu- 

 dinal folds. There are only two branchial slits, one on each side ; but 

 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, and 

 the line where the two tunics unite is marked by the so-called "gill" 

 (??); the posterior border of the slit is seen in Figs. 4, 32, and 33 as a 

 curved line passing by the anus and mouth, and then bending forward 

 to become continuous with the epipharyngeal fold (I), which bounds 

 the slit on the haemal side. Its anterior boundary is the peripharyn- 

 geal ridge, and it will be seen that the branchial sac is a complete 

 cylinder only in that short portion which lies anterior to this ridge ; 

 while from this backward to the mouth, its sides are entirely want- 

 ing, and the branchial cavity opens directly into the atrium. 



Although the inside of the branchial sac is supplied with cilia, as 

 in all Tunicates, these do not seem to be of as much functional impor- 

 tance as in the remaining members of the group, since respiration is 

 effected entirely through the action of the muscular girdles, which 

 also assist deglutition and are the organs of locomotion. These con- 

 tract rythmically, 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 allow its passage out of 

 the atrial aperture, but prevent its return. As the result of this 

 rythmical discharge of water, the animal is impelled forward with a 

 motion which is pretty uniform in the case of a perfect chain ; 



