386 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
zooids which are nearly uniformly distributed. Cloacal openings small 
round apertures numerously scattered over the surface" (Verrill, 
1871a). 
The right of this species to a place in this genus rather than in 
Trididemnum may perhaps be questioned, for on careful search, at 
least in some colonies, a few small calcareous bodies may be found in 
the test. The simplest and commonest ones are apparently nothing 
more than elongated crystals, but two of these may be joined near their 
middle points, forming a cross, or several or many may radiate from a 
common point, forming a stellate group (sometimes with rays of very 
irregular length, sometimes quite regular) which simulates, if indeed 
it does not sometimes actually become, a real spicule. Such groups 
are generally minute, usually attaining a diameter of only 0.02 nim. to 
0.04 mm., but occasionally much' larger 
ones occur, usually in company with 
smaller ones. In many specimens no 
calcareous bodies at all were found. In 
form the colony is usually expanded and 
incrusting, frequently not much over 
2 mm. thick, but occasionally S mm. or 
10 mm. in thickness. In outline it may 
he very irregular, and sometimes meas- 
ures 60 mm. or more in largest diameter, 
but usually very much less. Alcoholic 
specimens are grayish or yellowish white 
in color. 
The zooids vary much in size. As they 
are found in the preserved specifnens, few of them exceed 1.50 mm. to 
1.75 mm. in length, and a large majority are nuich smaller (1.15 mm. 
to 1.40 mm.), though in many cases such small size appears to be due 
to immaturity as well as to violent c(Mitracti(jn. In many cases the 
body is violently constricted or strangulated between the thorax and 
abdomen. The writer has not observed vascular processes extending 
out into the test, but is not prepared to say that they may not be devel- 
oped in certain stages of the life history of the colony. Neither is 
there a long muscular j^rocess as in many members of this family (as 
Didemnum [LcptocUnum] hifariiim), but the muscle bands running 
along the median dorsal vessel and to the posterior end of the endostyle, 
which are continuous with the muscles of the j^rocess in those forms 
in which it occurs, are visible in some individuals of the present spe- 
Text-fig. 16. — Diclemnopsis 
tenerum (Verrill). Crystalline 
bodies from the test of a colony 
from Eastport, Me. X 515. 
