INVEETEBRATE ANIMALS OF VINEYARD SOUND, ETC. 383 



danfc species is generally the Astyris lunata, (p. 306, Plate XXI, fig. 110,) 

 which generally occurs among the small algae and especially on the 

 Tubularians, in countless numbers; AnacMs avara (p. 306, Plate XXI, 

 fig. 109) is often found in considerable number ; Bittium nigrum (p. 305, 

 Plate XXIV, fig. 154) and Triforis nigrocinctus (p. 305, Plate XXIY, fig, 

 152) are usually common and the former often is very abundant ; Gerith- 

 iopsis Greenii (Plate XXIY, fig. 153) sometimes occurs, but is rare; Ilya- 

 nassa obsoleta (p. 354, Plate XXI, fig. 113) and Tritia trivittata (p. 354, 

 Plate XXI, fig. 112) are common, especially the former; Urosalpi-nx 

 cinerea (p. 306, Plate XXI, fig. 116) is generally to be found at or below 

 low- water mark on the piles and buoys; Bela plicata (Plate XXI, fig. 

 107) is sometimes met with, but is not common ; Odostomia bisuturalis 

 (p. 307, Plate XXIV, fig. 146) and other species of the genus are often 

 found near low-w T ater mark on the piles, especially where they are 

 somewhat decayed. Littorina palliata (p. 305, Plate XXIV, fig. 138) 

 and L. rudis (p. 305, Plate XXIV, fig. 137) nearly always occur near 

 high-water mark, on the piles, where there are alga?. In the harbors, 

 where the water is brackish, and less frequently in the purer waters, 

 the Alexia myosotis (Plate XXV, fig. 168) may be found on timbers and 

 piles near high-watermark, and sometimes, also, Skenea planorbis, (Plate 

 XXIV, fig. 142,) IAttorinella minuta, (Plate XXIV, fig. 140,) and Rissoa 

 aculeus, (p. 306, Plate XXIV, fig. 141.) Among and feeding upon the 

 Tubularians growing on the piles at and just below low-water mark, the 

 beautiful JEolidia pilata (Plate XXV, fig. 174) may often be found, espe- 

 cially in the harbors where the water is more or less brackish. 



Another related species, apparently the Gavolina gymnota, was found 

 by Professor Todd, on an old wreck in the Wood's Hole passage, but it 

 differs in several points from any form that has been described. The 

 branchiae were arranged in six transverse simple rows, on each side, 

 those of the second and third longest ; in the anterior rows there were 

 four to six branchiae, the lower ones much shorter than the upper ones. 

 In life the branchiae were dark green or blackish. 



Several other Gastropods are occasionally met with in these situa- 

 tions, but the species above named are about all that ordinarily occur. 



Among the Lamellibranchs, or " bivalve-shells," we find the Teredo 

 tribe, nearly all of which are peculiar to submerged wood-work, either 

 fixed or floating, and most of them are capable of doing great damage, 

 both to ships and to the timber and piles of wharves and bridges, or 

 other similar structures. Although popularly known as the " ship- 

 worm," these creatures are not at all related to the worms, but are true 

 mollusks, quite nearly allied, in many respects, to the common "long- 

 clam" {My a) and to the Pholas. Like those shells the Teredo excavates 

 its holes or burrows merely for its own protection, and not for food; but 

 the Teredo, selects wood in which to form its holes, and when these have 

 been excavated it lines them with a tube of shelly material. The holes 

 are very small at the surface of the wood, where they were formed by 



