VAN NAME: SIMPLE ASCIDIANS. 
529 
At once distinguishable from the other ascidians of this region 
(except the rare deep-sea Cw/^'o/(/.y sulimi, which the ordinary collector 
would not obtain) by having the body raised on a long slender stem. 
It was formerly made the type of a separate genus (Boltenia) on this 
account, but as shown by Hartmeyer (1903) the internal structure 
Text-flg. 24. — Pyura ovifera (Linnaeus). X 
is that of a Pyura closely allied to P. echinata. This species is widely 
distributed, and very variable in form and appearance, and several 
species have been established based upon these external differences. 
Hartmeyer (1903) after a careful study of many specimens came to 
the conclusion that these differences were individual peculiarities 
only, or due to age, and the writer after examining many specimens 
from different parts of the New England and Canadian regions has 
also failed to find difPerences indicating that more than one species 
is represented. 
Body of variable form, kidney -shaped, egg-shaped, or even spindle- 
shaped, sometimes decidedly compressed laterally, sometimes very 
little so, if at all. Apertures on widely separated papillae on the 
dorsal surface, both four-lobed, or the atrial aperture more or less like 
a transverse cleft. Stem arising from the anterior end or anterior 
ventral region of the body, generally about two to four times as long 
as the body; proportionately longer in rather small than in very large 
individuals. Its basal end is expanded for attachment to the rock 
or other object on which the animal grows. Its surface is usually 
transversely wrinkled. The test is very variable, sometimes thin, 
and but little if at all wrinkled on the surface, in other cases it is thick 
and deeply wrinkled or thrown into irregular elevations. In small 
and medium-sized specimens the surface both of the body and stem is 
often thickly covered with small thorn-like tapering processes, which, 
however, are soft and flexible, and not prickly to the touch. They 
