INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 401 



brown, finely mottled with white, and specked with darker brown ; 

 dorsal tentacles white and retractile ; lower surface white or light yel- 

 lowish, a three-lobed yellowish or brownish internal organ showing 

 through in the middle of the foot. Other specimens are very dark- 

 brown or almost black above, finely mottled with whitish. The ante- 

 rior angles of the head are prolonged into tentacle-like organs or palpi. 

 The gills are situated beneath, in the groove between the edge of the 

 foot and the mantle, on the left side, and near the posterior end of the 

 foot ; they consist of a tuft of slender filaments. 



Of Lamellibranchs certain species occur on rocky bottoms, which 

 attach themselves firmly to the rocks, either by the side of one valve, 

 like the oyster, Ostr^a Virginiana, (p. 310,) and the Anomia glabra, 

 (p. 311, Plate XXXII, figs. 241, 242 ;) or by threads of byssus,' which 

 they spin and use as cables for anchoring themselves, like the common 

 mnscle, Mytilus edulis, (p. 307, Plate XXXI, fig. 234,) the "horse- 

 muscle," Modiola modiolus, (p. 309, Plate XXXI, fig. 237,) the Argina 

 }}exata, (Plate XXX, fig. 227,) and Scapharca transversa, (Plate XXX, 

 fig. 228,) all of which are common in this region ; but certain other 

 species occur, which burrow beneath the stones, like the Saxicava arc- 

 tica (p. 309, Plate XXVII, fig. 192) and Mga arenaria (p. 463, Plate 

 XXVI, fig. 179,) and several other less common species. 



The Ascidians are usually very abundant on the rocks and stones at 

 all depths. The Cynthia partita (p. 311, Plate XXXIII, fig. 246,) is very 

 common, often forming large, rough clusters, much overgrown with hy- 

 droids, bryozoa, and alga\ The specimens mostly belong to the erect 

 variety, and in form are quite unlike the one figured. The body is 

 more or less cylindrical, oblong, or urn-shaped, about twice as high as 

 broad when expanded, and with a wide base ; the branchial orifice is 

 largest, and situated at the summit of a broad, terminal tube, swollen 

 at base ; the anal orifice is smaller, on a short lateral or subterminal 

 tube. Both orifices are usually squarish, and open widely, but, when 

 fully expanded, they sometimes become nearly circular ; they are often 

 surrounded at the edge with a narrow circle of red, and each tube has 

 eight longitudinal stripes of white, narrowing downward to a point at 

 the base of the tubes, and alternating with purplish brown ones, which 

 are usually specked with flake-white. The exterior of the test is more 

 or less rough and wrinkled, and generally yellowish or rusty, often 

 tinged with deep purplish brown on the upper parts or throughout. 

 The tubes are usually roughened by small, wart-like papilliie. Unprom- 

 ising as this species looks, it is devoured by the tautog. The Molgula 

 Manhattensis (p. 311, Plate XXXIII, fig. 250) is generally associated 

 with the former. The Ferophora viridis (p. 388) is often very abundant, 

 creeping over and covering up the two preceding, as well as other as- 

 cidians, algtie, hydroids, &c. The most conspicuous species, however, 

 are the massive compound ascidians, which sometimes completely cover 

 the bottom. One of the most abundant of these is the Amarwcium pel- 

 S. Mis. 61 26 



