VAN NAME: SIMPLE ASCIDIANS. 535 
branches arising alternately from each side of the stem. A few of the 
largest of these branches may bear slight projections along their side 
indicating incipient second-order branches. The third- and fourth- 
order tentacles can hardly be called branched; they have merely a few 
slight protuberances along the sides. The tentacles bear fairly well 
developed membranes. 
Dorsal tubercle large, C-shaped with spirally inrolled horns. The 
open interval was turned obliquely anteriorly and to the left in the 
specimens examined. 
Dorsal lamina broken up into a series of long tapering languets. 
The median dorsal vessel broadens out posteriorly and on the right 
side of the series of languets above mentioned there are smaller addi- 
tional languets (on the posterior portions sometimes as many as three 
or four abreast) . Endostyle stout and often thrown into lateral undu- 
lations in the preserved specimens. 
Branchial sac with eight folds on each side. The folds are very 
high, far exceeding in height the width of the interspaces, especially 
in large specimens, except the eighth fold, which is more or less rudi- 
mentary. The first fold is rather low, the second higher; the third, 
fourth, and fifth are highest. Transverse vessels very numerous, 
about five orders being recognizable in the ventral part of the sac, 
the smallest merely crossing the stigmata. In the dorsal part of the 
sac the fifth-order vessels are wanting and the fourth-order vessels 
are small and merely cross the stigmata. Internal longitudinal 
vessels rather close together, being generally separated on the inter- 
spaces between the folds by eight to twelve stigmata, while on the 
upper part of the folds they are closely crowded. A large number 
of stigmata (12 to 16) intervene between the endostyle and the nearest 
internal longitudinal vessel. Many of these vessels terminate before 
the posterior end of the sac is reached; those that extend the whole 
length end in a conspicuous recurved hook. They are almost as 
numerous on the dorsal as on the ventral leaves of the folds., The 
following is the approximate distribution of the vessels in a specimen 
about 35 mm. in height. 
mdv. 2 (16) 3 (18) 4 (22) 4 (24) 4 (22) 4 (14) 3 (10) 2 (3) 1 en. 
Smaller individuals usually have a less number. Stigmata placed 
longitudinally (their long diameter parallel to the internal longitudinal 
vessels). 
