VAN NAME: SIMPLE ASCIDIANS. 455 
attach themselves is evidently limited by that of the plant, which 
grows up during the summer months and dies down as cold weather 
comes on. 
Descriptions of Species. 
The numbered dredging and trawling stations mentioned are those 
of the United States Commission of Fish and Fisheries, (now the 
Bureau of Fisheries) and further data in regard to them (dates, 
temperatures, etc.) can be found in the Report of the Commissioner 
for 1886 (Washington, 1889). 
In classification and nomenclature the writer has followed the sys- 
tem of Hartmeyer as employed by that author in Bronn's Tier-reich, 
vol. 3, supplement, 1909. As a matter of convenience the order of 
families and genera employed in that work is followed here in nearly 
every case. Where this nomenclature involves the substitution of less 
known names for those which have long been commonly employed, 
the old name has in many instances been included in brackets after 
the new one thus: Caesira [Molgula] manhattensis (DeKay). 
The considerable number of species to be dealt with has made it 
necessary to restrict the discussion of the variations due to age, 
individual peculiarity, and many other factors, and to select for 
description and illustration adult and moderately large specimens 
exhibiting what appear to be average characters. But the reader 
should bear in mind that individual peculiarities are of frequent occur- 
rence in these animals, which though of complex structure, have no 
hard parts, and that in attempting to identify specimens, complete 
correspondence with the descriptions in every particular will be the 
exception rather than the rule. Such differences from the specimen 
described, as a differently shaped body, difference in coloration, a 
greater or less number of internal longitudinal vessels on the bran- 
chial sac, a smooth exterior in a species usually coated with sand 
or debris, or the opposite, must be expected to occur not only in single 
specimens but sometimes in all of a lot of individuals which are 
collected at the same time and place. 
The synonyms and literature of the several species that are given 
are chiefly those of American authors, those having some reference 
to the occurrence of the species in the waters of northeastern North 
America, or those of the original describer. For others having refer- 
ence to their occurrence in foreign waters the reader is referred to the 
