294 BULLETIN OF THE 



sac, and therefore opposite the endostyle, there is a row of tongue- 

 shaped organs projecting into the respiratory cavity ; these are the 

 " languettes." The number of these is very variable ; in Salpa there 

 is only one (Figs. 1 and 3, w), in Pyrosoma there are eight, and in 

 most of the fixed ascidians the number is much greater. 



At the posterior end or base of the branchial sac, its wall is directly 

 continued into that of the oesophagus (Fig. 24, o',) which is short and 

 opens into a somewhat dilated stomach (Figs. 24, a", and 4 o"). The 

 cavities of these organs are continuous with the branchial cavity, and 

 since the branchial sac is shown, by its development, to be nothing 

 more than the anterior end of the digestive tract, Huxley has called 

 it the " pharynx" ; but it will be much more convenient, for our pur- 

 pose, to retain the name which indicates its functional but not its 

 morphological relations. The anterior opening, already described as 

 the " branchial aperture " is undoubtedly the true homological 

 mouth, but it is much more convenient to make use of the name 

 which indicates its function, and to apply the term " mouth " to the 

 aperture of ingestion, at the point where the oesophagus leaves the 

 base of the branchial sac. The intestine bends around the stomach 

 and then runs forward nearly parallel with the oesophagus (Fig. 24), 

 and usually terminates at a point which is anterior to the mouth ; 

 the anus opens into the cavity of the " atrium," which is now to be 

 described. 



The Atrial Tunic. — Since the form and relations of this structure 

 are very variable in the different Tunicata, a clear conception of it as 

 presented in the typical forms is essential to a correct appreciation of 

 tunicate structure and development in general, and, owing to a lack of 

 such a clear conception, many of the published accounts of these ani- 

 mals are of very little value. In the perfectly transparent genus 

 Perophora, which presents exceptionally favorable conditions for the 

 study of the atrium, the two openings of the external tunic are at the 

 same end of the body ; one of these, the branchial aperture, is formed 

 by the union of the branchial sac and outer tunic, as already de- 

 scribed, while the other, the atrial aperture, is formed in a similar way 

 by union of the outer and atrial tunics. The latter is a large bag, 

 parallel to the branchial sac, and upon that side of it which bears the 

 languettes ; at the bottom of this sac, which Huxley has called the 

 " mid-atrium," are the external openings of the intestine and repro- 



