BY H. LEIGHTON KESTEVEN. 279 



thorax is fairly constantly 2*5 mm., that of the abdomen half of 

 this, and of the postabdomen a little more than half this again. 

 The length of the postabdomen is variable. In preserved speci- 

 mens the six-lobed oral siphon (PI. xxv., fig. 9) is found to be 

 well, though not deeply retracted; the simple atrial aperture, 

 nearly level with the oral, is probably much nearer its normal 

 position. The vascular appendages are simple tubular unbranched 

 structures. 



The common cloacal chambers are small and flattened in the 

 plane of the surface of the colony. From each chamber a variable 

 number of tubular diverticula extend between the ascidiozooids to 

 their respective atrial siphons. Those to the nearest individuals 

 take a nearly straight course; to those more distant the tubules 

 radiate out parallel and close to the surface of the test, and then 

 dip directly down to their destination. These tubules, though 

 composed of test, lined by an epithelium, are not merely excavated 

 in the test, but lie more or less free and often come away attached 

 to the siphon (see fig. 9). 



Systems, though undoubtedly present, are so variable as to be 

 undefinable; there were apparently ten individuals in one system 

 counted. 



The test, owing to its strengthening of sand-grains, is firmer 

 on the surface than within; in consistency it is intermediate 

 between cartilaginous and gelatinous; and, owing to the close 

 packing of the ascidiozooids, which reduces the test to a minimum, 

 it may be aptly described as membranous. The cells are very 

 numerous, and of two kinds. The more numerous form (Plate 

 xxvi., fig.3) has an approximately oval shape rendered irregular 

 by the expanded bases of extremely fine dendritic processes of 

 great length, given off from both ends. These processes are very 

 occasionally seen after staining with eosin, but, on using Van 

 Gisson's picro-acid fuchsin, they show up very distinctly, and by 

 a sufficiently high magnification the test is shown to be pervaded 

 with these proces.ses; their tenuity is of about the same order as 

 that of white fibres of mammalian areolar tissue. Cells of the 

 second form are of the usual spindle type; a few of the normal 



