[401] INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 107 
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, Ostra 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 
muscle, Mytilus edulis, (p. 307, Plate XXXI, fig. 234,) the ‘“ horse- 
muscle,” Modiola modiolus, (p. 309, Plate XX NI, fig. 257,) the Argina 
pexata, (Plate XXX, fig. 227,) and Scapharea 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 are- 
tica (p. 309, Plate XXVII, fig. 192) and Mya 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 
alldepths. The Cynthia partita (p. 511, Plate XX XIII, fig. 246,) is very 
common, often forming large, rough clusters, much overgrown with hy- 
droids, bryozoa, and alge. 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 papille. Unprom- 
ising as this species looks, it is devoured by the tautog. The Molgula 
Manhattensis (p. 311, Plate XXXII, fig. 250) is generally associated 
with the former. The Perophora viridis (p. 588) is often very abundant, 
creeping over and covering up the two preceding, as well as other as- 
cidians, algve, hydroids, &c. The most conspicuous species, however, 
are the massive compound ascidianus, which sometimes completely cover 
the bottom. One of the most abundant of these is the Amaracium pel- 
9 V 
