VAN NAME: SIMPLE ASCIDIANS. 461 
compressed laterally. In other cases the siphons may remain ex- 
tended, in which case the modified area about their bases described 
above is often inconspicuous. 
Transverse muscle fibers prevail in the mantle and are for the most 
part gathered into very slender bands which are not at all conspicu- 
ous on most parts of the body. But in the vicinity of the siphons 
strong bands spread out in a radial manner over the body surface 
becoming abruptly slender after a certain distance, and near each side 
of the mid-ventral line (though not extending across that line) there 
are many stout transverse muscles bands. The circular muscles 
about the bases of the siphons are also well developed. 
The tentacles are numerous and of about four different sizes or 
orders arranged with some regularity, the largest numbering about 
eight. The largest tentacles are irregularly bipinnately or to some 
extent tripinnately branched. Those of intermediate size are once 
or twice pinnate with a few rather irregular branches; the smallest 
are entirely unbranched. The branches terminate in blunt, scarcely 
enlarged tips. 
The orifice of the dorsal tubercle is C-shaped or horseshoe-shaped 
with the open interval directed toward the left. In large specimens 
the horns are generally more or less irregularly incurved or bent. 
The dorsal lamina is plain-edged. 
The branchial sac is entirely without folds, but has in their place 
seven very stout internal longitudinal vessels on each side which are 
crossed by five transverse vessels, dividing the sac into 48 fields on 
each side of the body. The stigmata are very long and arranged 
in chains of double spirals, and the part of the wall of the branchial sac 
occupied by each well developed spiral becomes raised into an inter- 
nally projecting infundibulum or cone, upon whose surface one of the 
long stigmata winds from the base to the apex and terminates there. 
Another stigma winding down to the base between the coils of the first 
stigma, crosses to an adjacent infundibulum upon which it forms the 
ascending spiral, terminating at the apex. Thus the spirals of a 
number of infundibula form a connected series or chain, and would, 
were it not for the interruptions at the apices of the infundibula, 
consist of one continuous long stigma. The infundibula, which are 
often very long, forming somewhat curved finger-shaped projections 
into the branchial cavity of various sizes and lengths, are numerous 
and show no noticeable regularity in their arrangement in large indi- 
