VAN NAME: SIMPLE ASCIDIANS. 463 
anterior and posterior to the primary spiral, but usually do not form 
on the area dorsal to it except in old and large individuals when the 
secondary spirals are already numerous. As far as the writer has 
observed, the system of connected spirals of which one primary spiral 
is the center, is in normal cases totally distinct from those connected 
with any other primary spiral. In large individuals, independent 
systems of spirals form in the interspaces of the principal chain, and 
increase the complication of the arrangement. Delicate radial ves- 
sels, usually four in number, crossing without interrupting the stig- 
mata strengthen and support each infundibulum. 
Not only the distribution, but the direction of twist of the spirals 
is subject to a definite scheme in young individuals. Those primary 
infundibula (with the exception of those dorsal to the first, and those 
bridged over by the seventh internal longitudinal vessel) which are 
situated in the same vertical row have the same direction of twist. 
The direction is different in alternate vertical rows. Assuming that 
the twist is from the base toward the apex, the stigmata curve in the 
dorsal half of each such primary infundibulum toward the odd-num- 
bered transverse vessels, and the free ends of the stigmata which ex- 
tend from them (or from the chain of stigmata of which they form a 
part) lie close to the transverse vessels having odd numbers. This 
is shown in figs. 3 and 5 (Pis. 43, 44) where comparatively simple 
arrangements of the spirals are represented. The secondary spirals 
first developed in each chain and a majority (but not all) of those 
formed later generally twist in the opposite direction to their primary 
spiral. 
Save for occasional irregularities and individual peculiarities, the 
above described arrangement prevails throughout the greater part 
of the sac. The seventh or most ventral internal longitudinal vessel 
differs in having two primary spirals under (bridged over by) each 
of its segments. These spirals twist in opposite directions curving 
toward each other in the dorsal half of each spiral, as in the ordinary 
primary spirals separated by an odd-numbered transversa vessel. 
These spirals are connected together through the secondary spirals, 
lying ventral to them and other secondary spirals develop dorsal to 
them even in young animals. 
In some indi\iduals merely irregularly curved and hooked stigmata 
occur in some of the fields along the median dorsal vessel. In other 
individuals well formed spirals exist, often having a direction of twist 
opposite to that normal in the vertical column in which they are 
situated. 
