ASCIDIAN. 203 



But in the genus Cynthia they are disposed in two layers, an outer longitu- 

 dinal and an inner circular layer. 



The cavities of the pharynx and digestive tract have been displayed by 

 the removal of their own and the body walls on the left side. Close to the 

 margin of the oral aperture may be noticed, first, the circle of tentacles 

 which guard the entrance, secondly, behind this circle two delicate bands, 

 the peripharyngeal bands which run circularly round the pharynx, and 

 thirdly, the cavity of the pharynx itself. This cavity is marked by trans- 

 verse and longitudinal lines at right angles to one another, inclosing 

 innumerable square meshes. The transverse lines are the transverse vessels 

 which run from the ventral to the dorsal edge of the pharynx, connecting 

 the two main longitudinal bloodvessels which correspond to these two edges 

 respectively. The longitudinal lines are the longitudinal bloodvessels which 

 run on the inner surface of the pharyngeal cavity. They extend from the 

 anterior to the posterior end of the pharynx, and are connected by short 

 vessels to the transverse vessels at the spot where the two sets Cross one 

 another. The longitudinal vessels which pass from one transverse vessel 

 to another, and lie in the same plane as they do, are very fine and inclose 

 the stigmata or apertures which lead from the cavity of the pharynx to the 

 peribranchial or atrial cavity. They are not visible without the use of the 

 microscope. The cilia which beset the stigmata cause a current of water to 

 flow from the pharyngeal into the atrial cavity. The pharynx has thus a 

 respiratory function and is hence often termed branchial sac. 



The pharynx extends on the right side of the animal to its posterior 

 extremity, and the digestive portion of the alimentary canal is placed to its 

 left side, an arrangement which does not occur in all Urochorda, e.g. Cla- 

 velinidae. Stretching along the dorsal edge of the pharyngeal cavity is a 

 delicate longitudinal band, the dorsal lamina. Anteriorly it may be seen 

 to come into contact with the peripharyngeal bands. At this point there is 

 an opaque white spot, the nerve-ganglion which is underlaid by the neural 

 gland, and in front of the opaque spot may be noted a crescentic curved 

 line with .the concavity turned forwards. This line represents the dorsal 

 tubercle, so-called, or the aperture of the neural gland. When the specimen 

 is held in a certain position as regards the light, a yellow opaque line may 

 be seen running along the ventral edge of the pharynx internally. This line 

 is the endostyle, which extends from the peripharyngeal bands anteriorly to 

 the posterior end of the dorsal lamina, close to which is the aperture from 

 the pharynx into the stomach. A black bristle has been passed down the 

 pharynx and through this aperture. 



The pharynx is attached to the body walls along thejine of the endo- 

 style, but a cavity, the atrial or peribranchial cavity, which opens externally 

 by the atrial aperture, extends round its dorsal edge throughout nearly its 

 whole extent to the right and left sides, separating them from the body 



