484 THE AMERICAN NATURALIST. [Vor. XXXV. 
from the body wall and is attached at the end to the radial 
piece of the calcareous ring. By their contraction these five 
muscles serve to draw the tentacles and whole anterior end of 
the animal back into the body cavity for protection, and they 
are therefore called the retractor muscles. 
Owing to the fact that the early describers of holothurians 
paid little attention to internal characters, and none whatever 
to the microscopic deposits in the skin, it is difficult, if not 
impossible, to determine from one of their descriptions what 
species is meant. Even within the past forty or fifty years so 
much importance has been attached to those characters in 
which there is the greatest individual diversity (size, shape, 
color, number of tentacles, arrangement of pedicels, etc.) that 
the same species has been described under a number of dif- 
ferent names. Many of our common species have two or three 
synonyms, some have four or five, and one has no less than 
eleven! As very few types of the species described by the 
earlier writers are now in existence, the untangling of this 
snarl of synonyms is no easy task. No less than eighty-eight 
different specific names have been given to holothurians from 
thé shallow waters of North America; but these seem to be 
reducible to at most forty-seven valid species, and very probably 
the correct number now known is under forty-five. Since 
many of the names in most common use in this country are 
incorrect, they will be looked for in vain in this key. In order 
that such forms may be properly placed, the following list of 
synonyms is given. It is not intended as a complete synonymy 
of the species given in the key, but is simply an alphabetical 
list of names incorrectly applied, or no longer in use by the 
best authorities. 
There are a number of such generic names, as follows : 
Actinopyga Brown = Miilleria of the key. 
Anaperus Troschel = Thyone "um 
Botryodactyla Ayres = Cucumaria “ “ «“ 
Cladodactyla Brandt = Cucumaria “ “ + 
- 
Embolus Selenka = Trochostoma * * «+ 
Echinosoma Semper = Eupyrgus à uc 
Leptosynapta Verrill = Synapta 64 d “ 
