46 BRITISH TUNICATA. 



and the space within is divided into pouches by septa situated 

 at nearly equidistant points. In this genus the transverse 

 vessels vary considerably in size, there being- usually one or 

 two smaller between the larger ones ; and the septa are placed 

 wherever the latter cross the structure. Thus a series of 

 pouches of nearly equal size occupy the interior of the folds, 

 and open at the outer surface of the branchial sac into every 

 part of the atrium. In fact, we see here an arrangement very 

 similar to that observed in the interbranchial water-channels 

 in the Laniellibranchs ; and in this case, as in them, the pur- 

 pose is to allow the water, after permeating the walls of the 

 fold or lamina, to escape externally. 



The longitudinal bars in Btyela assume the form of delicate 

 ribbon-like membranes attached by one edge to the principal 

 transverse vessels; they are numerous, and are found on the 

 folds as well as on every other part of the organ. The oral 

 lamina is a wide simple membrane. 



The branchial sac in Mongula conchilega and its allies is 

 characterized by longitudinal folds or laminae, formed much 

 in the same way as those in Stt/ela. In the former there are 

 six such folds on each side of the sac. The vascular network, 

 however, is very different, having the secondary vessels, or 

 those which are usually arranged at right angles to the trans- 

 verse channels, disposed in imperfect spiral coils or convolu- 

 tions, the vessels themselves frequently intercommunicating ; 

 consequently the stigmata or open spaces separating them are 

 broken into various lengths. There are also a few delicate 

 radiating vessels which pass from the centre of the coils to 

 the circumference, but mostly in the direction of the trans- 

 verse channels that convey the blood to and from the coils. 

 The chief purpose of these radiating vessels is apparently to 

 prevent engorgement of the coil, and to aid the reflux of the 

 stream by conveying the blood in the most direct manner to 

 and from the centre of the coil. When the spiral arrange- 

 ment is more imperfectly developed than usual, the radiating 

 vessels are very irregularly dispersed ; but even in such cases 

 there can be little doubt that their function is as above stated. 

 The longitudinal bars have the ribbon-like character of those 

 in fttyela, and are principally confined to the folds. 



Corella parallelogr&mma has also the secondary vessels 

 spirally arranged, as originally pointed out by Mr. Alder* ; 

 and here the coils, which form slight conical eminences,t are 



* "Observations on the British Tiinicata," 'Ann. Nat. Hist.,' ser. 3, xi, 

 p. 158. 



f [Op. cit., pi. vii.] 



