-5^2 STUDIES ON TUNICATA, 



-numerous, spherical or spherical-crenated without vacuoles, or 

 irregularly distended by vacuoles. The ectoderm adheres closely 

 to the test, and a thin strip taken at random from the inside 

 always bears several patches. The phagocytes (?) take a darker 

 stain than the squames of the epithelium, and may be clearly seen 

 among them, as well as throughout the test. Large bladder-cells 

 are present, and some of these certainly have the appearance of 

 being filled with smaller cells. Typical vessels are not well 

 developed, and one of the preparations made shows the presence 

 of peculiar branched vessels of small lumina, and lined by h'gh 

 columnar epithelium. 



The mantle is strong and semitransparent; it adheres ^ery 

 closely to the test. Fine retractor bands are developed round 

 both siphons (Pl.xxvii., fig.l), and muscle-fibres are matted 

 throughout the whole mantle. 



The branchial sac (PL xxvi., fig. 7) is weak though fairly thick^ 

 and is difiicult to detach from the mantle intact. There are four 

 strong folds on either side; their relation to endostyle, dorsal 

 lamina and the two apertures is shown diagrammatical ly in 

 fig. 5, Plate xxvii. The rear ethree internal longitudinal vessels 

 in the interspaces, four on the sides and one on the crest of each 

 fold. Transverse vessels are of very various sizes (PL xxvi., fig. 7) 

 and irregular arrangement. Where most regularly arranged^ 

 three sizes can be recognised; between each pair of the largest, 

 are three of medium size, and alternating with them are those 

 of the smallest size. In places these last are represented by 

 anastomoses of the interstigmatic vessels, rather than by definite 

 vessels, and in other places are not developed at all. Occasionally 

 the interstigmatic vessels are very much enlarged; such enlarged 

 vessels may extend across three or four meshes. Fine membranes 

 are present along a few of the transverse vessels. There are 

 about twelve stigmata to a mesh near the base of the folds, and 

 about six on either side of the crest. On either side of the 

 endostyle the meshes are very broad and contain upwards of 

 twenty-five stigmata. The interstigmatic vessels are rather finer 

 than the stigmata. 



