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THE ECHINODERMS OE THE VVOODS HOLE REGHOIV. 
ECHINODERMATA. 
Radially symmetrical animals, with a well-developed water-vascular system. 
Key In the classes of Echinodermala of the Woods Hole region. 
Body more or less flattened dorso-ventrally, with the mouth at the center of the lower surface; external skeleton well 
developed. 
Body with the radii extended as more or less elongated arms. 
Arms unjointed, with a longitudinal furrow on the lower side, in which are the numerous tube feeh . ^Asteroidea 
Armsi made up of numerous joints, without a longitudinal furrow on the lower side Opiiiuroidea 
Body hemispherical or discoidal, without extended radii Echinoidea 
Body more or less elongated, with the mouth at the anterior end, surrounded by a circle of tentacles; 
no well-developed external skeleton a.... i HoLothurioideA. 
ASTEROIDE A. 
STARFISHES. 
Of the 6 starfishes which occur in the Woods Hole region, no fewer than 4 belong 
to the large and widely distributed genus Asterias, the species of which are, even to 
the present day, greatly confused. For this reason it is impossible to avoid the use. 
of some technical terms in the artificial key, as well as in the descriptions, although 
the endeavor has been made to have the descriptions as clear and free from techni- 
calities as possible. 
Following are the terms used which do not carry their own meaning: 
Ahacti,nal=a.hcmi\; opposite the mouth; the upper surface. 
Ac.tinal= oral ; the lower surface. 
Adambulacral spines = the spines borne on the plates which form the margin of the aetinal longitudinal 
furrow on each arm. 
A mhulacral farrow =the act.inal longitudinal furrow on each arm. 
Madrepore plate= the more or less prominent somewhat circular body situated abactinally in one of 
the interradii. 
Oral plates=the skeletal plates, especially the adambulacral plates, immediately surrounding the 
mouth. 
Papulie—Xho. tentacle-like outgrowths of the body wall which project between the meshes of the 
skeleton and contain prolongations of the body cavity. 
Papular areas=spaces occupied by papulae between the meshes of the skeleton. 
Pedicellarise= the minute forcep-like modified spines, consisting of two hard jaws moved by muscles, 
and occurring in many starfish, especially on the abactinal surface, and at the base of the 
large spines. 
It must be added, moreover, that no description or figures will serve to distin- 
guish positively very young individuals of our 4 species of Asterias. The young of 
forbesi seem to be the first to show the specific characters, and individuals 15 mm. in 
diameter can generally be recognized without much trouble. The young of vulgaris , 
ienera , and austera resemble each other so closely when less than 10 mm. in diameter 
that it is practically impossible to separate them positively, hut when 15 mm. in diam- 
eter they can usually be distinguished from each other on careful comparison, and 
by the time they have reached a diameter of 20 mm. the specific characters are 
generally quite marked. It must be borne in mind, however, as shown by Mead (’99), 
that the age of a starfish is not shown by its size, and, furthermore, that spines and 
pedicellarhe increase in number as the age increases. Thus some small but old 
specimens may have the specific characters well defined, while others much larger 
but younger may have them barely indicated. 
