VAN NAME: SIMPLE ASCIDIANS. 503 
siphons are few in number and extend but a little way on the sides, 
though farther on the dorsal region of the body between the siphons 
and along the mid-ventral region. Slenderer and less regular circular 
bands cross the radiating bands, and between the siphons and along 
the ventral region near the endostyle slender, more or less irregular 
transverse bands are numerous and quite closely placed. 
At least five orders of oral tentacles are present, the largest of which 
are extensively branched in an irregular tripinnate manner and 
usually number six, while those of the second and third orders are also 
much branched; those of the remaining orders are small and com- 
paratively or entirely simple. Their arrangement is fairly regular 
according to the usual scheme (1, 5, 4, 5, 3, 5, 4, 5, 2, 5, etc.). 
Dorsal tubercle generally with a plain C-shaped opening; the horns 
not spirally rolled. They usually turn to the right. 
Dorsal lamina plain for much of its length. Posteriorly it becomes 
very broad and bears small triangular teeth. Similar but longer and 
narrower teeth (only four or five in number) occur on the extreme 
posterior end of some of the internal longitudinal vessels. 
Branchial sac of delicate structure with six folds on each side. 
These folds are but slightly prominent and each might perhaps be 
better described as a row of infundibula bridged over longitudinally 
from a group of internal longitudinal vessels. Except as noted below, 
these vessels are confined to the folds and numbered on the right side 
of two fairly large individuals of average characters about as follows: 
mdv. (7) (6) (6) (5) (4) (4) en. 
mdv. (7) (S) (7) (6) (5) (4) en. 
Owing to the slight prominence of the folds nearly all these vessels 
are visible as the folds lie in their normal position. In some specimens 
one of the vessels of the group belonging to a fold often lies on the 
interspace somewhat removed from, and to the dorsal side of the base 
of the fold. This was observed in the case of the four first (most dorsal) 
folds in one individual. In other individuals these isolated vessels, 
if any are present, are fewer and very rudimentary and irregular. 
The large transverse vessels of the sac, extending from the median 
dorsal vessel to the endostyle apparently represent two sizes or orders, 
the normal number of the largest ones being five. There is, however, 
often so little difference between those of the first and the second order 
that they are hard to distinguish, and in the ventral part of the sac 
