INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 305 



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 regularity 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 for respiration. 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." One of these (Plate XXIV, fig. 138) 

 is subglobular in form, the spire being depressed and the aperture wide. 

 This is the Littorina 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 and 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 XXIY, fig. 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 the algoe among which 

 they live. Another allied shell, the Lacuna vincta, (Plate XXIY, 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, and most commonly is encircled by two or more 

 darker, chestnut-colored bands. This also feeds upon the alga?. 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 XXIY, 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 

 S. Mis. 61 20 



