372 PEOCEEDINGS: BOSTON SOCIETY NATURAL HISTORY 
Text - fig. 8. — Didcmnum 
lutarium, n. sp. [Spicules. 
X 515. 
across and from 2 to 4 mm. thick, or even thicker in hirge specimens. 
As compared with that of Tetradidemnum albidum, the only species of 
this region with which it is hkely to be often confused, the common test 
substance is softer and much more easily broken, and owing to the 
minute size of the spicules the surface does 
not have such a rough, gritt}' character. It 
is, however, commonly more wrinkled than 
in that species, and the furrows often appear 
to indicate in a rough way the forms of the 
systems in which the zooids are arranged. 
The small rounded elevations over the zooids, 
which are so noticeable on the surface of 
most specimens of T. alhidxim, are usually 
not developed in this species, and the 
branchial orifices are usually conspicuous. 
The color is sometimes a chalky white, but often it is tinged with yellow 
or reddish. When the colony grows in water that is even slightly mtiddy 
it becomes more or less discolored with the mud. The spicules are 
exceedingly numerous and thickly distributed 
in most parts of the test in most specimens. 
They are for the most part under 0.02 mm. in 
diameter and usually have a rather small num- 
ber of regularly tapeiing conical points with 
moderately acute tips. 
The zooids are so minute that their structure 
is by no means easy to make out. In the con- 
tracted state they are often under 1 mm. in 
length. Their bcjdy is short, and sharply con- 
stricted between the thorax and abdomen. 
I'here are longitudinal mtiscle bands in the 
mantle on the thorax, and a stout muscular 
process of moderate length extending out into 
the common test from the middle region of the 
body. 
The branchial orifice has six very short lobes; 
jilain opening without a languet. The tentacles are apparently 16 
in number, of two sizes placed alternately. The branchial sac has 
10 or 11 stigmata in a row on each side, in the middle rows at least. 
The posterior row probably has a less number. The stomach is 
Zooif 
X 4S. 
the atrial orifice is a 
