[305] INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 11 
it quickly sweeps forward with a grasping motion and then quickly 
withdraws, as if in search of food, and this motion will be repeated with 
great recularity for a long time, unless the creature be disturbed, when 
it instantly withdraws its net and closes its doors. No one who will 
take the trouble to examine this little animal, when in active operation 
in one of the tide-pools, can fail to admire its perfect adaptation to its 
mode of life and the gracefulness of its motions. The movement 
referred to serves not only to obtain food, which, in the form of micro- 
scopic animals, is always abundant in the water, but also to supply 
fresh currents of water forrespiration. This creature is also well worthy 
of mention here because it serves as food for the tautog, and probably 
for other fishes that can obtain it at high water. 
Two species of small univalve shells (Littorina) are always to be found 
in abundance clinging to the surface of the rocks, or among the sea- 
weeds, or creeping about in the tide-pools. These are often found quite 
up to high-water mark, but the full-grown ones are more common lower 
down among the “rock-weeds.” Oue of these (Plate XXIV, fig. 138) 
is subglobular in form, the spire being depressed and the aperture wide. 
This is the Litiorina palliata. It varies much in color; the most com- 
mon color is dark olive-brown, not unlike that of the Fucus, but orange- 
colored and pale yellow specimens are not uncommon, while others are 
mottled or banded with yellow or orange aud brown. The second spe- 
cies is more elongated and has a more elevated and somewhat pointed 
spire. This is Littorina rudis, and it has many varieties of form, color, 
and sculpture; one of its varieties is represented on Plate XXIV, tig. 137. 
Some specimens are smooth, others are covered with revolving lines or 
furrows; in color it is most frequently dull gray, olive-green, or brown, 
but it is often prettily banded, checked, or mottled with yellow or orange, 
or even black, and sometimes with whitish. This species is viviparous. 
These shells are both vegetarians and feed upon tbe alge among which 
they live. Another allied shell, the Lacuna vincta, (Plate XXIV, fig., 
139,) is found clinging to the sea-weeds at low-water mark and some- 
times in the tide-pools. This is usually pale reddish or purplish 
brown, or horn-colored, aud most commonly is encircled by two or more 
darker, chestnut-colored bands. This also feeds upon the alge. Asso- 
ciated with the last, two or three other kinds of small shells are gener- 
ally found. One of the most abundant of these is the Bittium nigrum, 
(Plate XXIV, fig. 154,) which is, as its name implies, generally black, 
especially when young, but large specimens are often only dark brown 
or even yellowish brown below ; it occurs in great abundance, clinging 
to the sea-weeds and eel-grass at and below low-water mark, and is also 
to be found in the tide-pools and on the under sides of rocks. Associated 
with the last, and resembling it in form and color as well as in habits, 
another much less common species occurs, which is remarkable for hav 
ing its whorls reversed, or coiled to the left, in the direction opposite to 
that of most other shells. This is the Triforis nigrocinctus, (Plate 
