490 THE AMERICAN NATURALIST. [Vor. XXXV. 
Holothurians are easily distinguished from any other inver- 
tebrates by the anterior and usually terminal mouth, with a 
circumoral ring of tentacles, and their curious combination of 
radial and bilateral symmetry. Although the bilateral sym- 
metry seems usually to predominate, there are always five 
prominent longitudinal muscles accompanied by nerve strands 
and vessels of one or more sorts. The body wall is soft or 
Fics. 1-4. 
1. Trochostoma boreale (Sars). Nat. size. 3. Psolus chitonoides Clark. N 
2. Cauds: ¥ (G ld) at. siz 
at. size. 
4. Psolus phantapus (Struss.). $ nat. size. 
leathery, and generally contains calcareous particles in greater 
or less abundance. These particles are usually microscopic 
but are occasionally in the form of large plates several milli- 
meters across. Their size, shape, and arrangement differ to 
an extraordinary degree in the various genera and species, and 
they form one of the most satisfactory means of identification. 
In their simplest condition these calcareous deposits take the 
form of very minute, more or less irregular grains known 4S 
