INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 361 



ward motion, is obtained. It is a singular sight to see this shell swim 

 swiftly many times around a vessel of water, at the surface, until, 

 finally, becoming exhausted by its violent exertions, it sinks to the 

 bottom €or rest. 



The common " scollop," Pecten irradians, (Plate XXXII, fig. 243,) is 

 also frequently found living on sandy shores and flats, or in the pools? 

 but it belongs more properly to the sheltered waters of the ponds and 

 estuaries, where it lives among the eel-grass. It will, therefore, be 

 mentioned again in that connection. 



The " common muscle," Mytilus edulis, (p. 307,) is frequently found in 

 large patches on sandy flats, fastened together by the threads of 

 byssus. Some of the most beautifully colored varieties, (fig. 234,) with 

 radiating bands of blue and yellow, are often found in such places, but 

 the species is much more abundant and larger in other situations, 

 especially in the shallow and sheltered waters of the bays, where there 

 is more or less mud. 



Ascidians are almost entirely wanting on the sandy shores, but Mol- 

 gula Manhattensis (p. 311, Plate XXXIII, fig. 250) is sometimes found 

 even on sandy shores, attached to eel-grass. 



Of Bryozoa ouly two species are usually met with, and even these do 

 not have their true stations on the sandy shores. The delicate and 

 gracefully branched Bugula turrita (p. 311, Plate XXXIY, figs. 258, 

 259) is occasionally found growing attached to the eel-grass, which 

 often grows in the sandy tide-pools, or at extreme low- water. It also 

 occurs in great abundance among the masses of sea-weeds thrown up 

 by the waves on the sandy beaches. Such specimens are often large 

 and luxuriant, in some cases being more than a foot in length ; these 

 are derived from the bottom in deeper water, off shore. 



The Escliarella variabilis (p. 312, Plate XXXIII, fig. 256) is often found 

 encrusting dead shells of various kinds, especially such as are inhab- 

 ited by the larger "hermit-crabs." It is also cast up in abundance, on 

 some beaches, from deeper water. 



The Radiates are not numerous on sandy shores, yet several interest- 

 ing species may be found. Among the Echinoderms we find four 

 species of holothurians, one sea-urchin, one star-fish, and one ophiuran. 



The most common holothurian is the Lejptosynapta Girardii, (Plate 

 XXXY, figs. 265, 266.) This is a long, slender, very delicate and fragile 

 species, which burrows deeply in the sand or gravel near low-water 

 mark. The holes are round and go down almost perpendicularly ; 

 they are usually not more than a quarter of an inch in diameter. The 

 creature is not quick in its motions, and can usually be found in the 

 upper part of its burrow when the tide is out. The skin is thin and 

 quite translucent, so that the white muscular bands that run lengthwise 

 of the body, on the inside, can be easily seen, as well as the large intes- 

 tine, which is always quite full of sand 'and gives a dark appearance to 

 the body. The tentacles are almost always in motion, and are used in 



