INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 367 



a certain extent, conditions similar to those of rocky shores, and conse- 

 quently it is natural that certain rocky-shore species should be found in 

 such situations. Only the more common and most important of these 

 have been introduced into the list, however, for to include all the species 

 to be found among oysters would uselessly extend the catalogue. 



Among the Crustacea we find a considerable number of species which 

 have their proper homes on the muddy shores. Of the true crabs there 

 are at least eleven. species that constantly occur in these situations, but 

 several of them, viz., Cancer irroratus, (p. 312,) Panopeus depressus, (p„ 

 312.) P. Sayi, (p. 312,) and Carcimis granulatus, (p. 312,) are found in 

 greater numbers elsewhere, and depend largely upon the oyster-beds for 

 their safety on these shores. The Carcinus granulatus, however, often 

 resorts to the holes and cavernous places under the peaty banks of the 

 shores, or along the small ditches and streams cutting through the peaty 

 marshes near the shore. The marsh " fiddler-crab," Gelasimus pugnax, 

 is usually very abundant in the peaty banks and along the ditches and 

 streams at and just above high-water mark, where it excavates great 

 numbers of deep holes, often completely riddling the soil. This species 

 is, however, more at home along the borders of the estuaries and 

 lagoons and will be described more fully in that connection, as well as 

 the Sesarma reticulata, which often occurs with it in both situations. 



The " oyster-crab," Pinnotheres ostreum, (Plate 1, fig. 2, male,) is found 

 wherever oysters occur. The female lives, at least when mature, within 

 the shell of the oyster, in the gill cavity, and is well known to most con- 

 sumers of oysters. The males (fig. 2) are seldom seen, and rarely, if ever, 

 occur in the oyster. We found them, on several occasions, swimming 

 actively at the surface of the water in the middle of Vineyard Sound. 

 They are quite unlike the females in appearance, being smaller, with a 

 firmer shell, and they differ widely in color, for the carapax is dark brown 

 above, with a central dorsal stripe and two conspicuous spots of whitish, 

 as indicated in the figure ; the lower side and legs are whitish. The 

 female has the carapax thin and translucent, whitish, tinged with pink. 

 The Pinnixa cylindrica (Plate I, fig. 1) is a related species which is 

 occasionally met with on muddy shores. It lives in the tubes of certain 

 large Annelids in company with the rightful owner. The specimens 

 hitherto met with in this region were either found free, or dug out of 

 the mud, and it is uncertain with what worm they associate, though it 

 is most likely to be the Nereis virens, but on the coast of South Carolina 

 it lives, according to Dr. Stimpson, in the tubes of Arenicola cristata 

 Stimpson. It has been found in the stomach of the ocellated flounder. 

 The common edible-crab or " blue-crab," Callinectcs Jiastatus, is a com- 

 mon inhabitant of muddy shores, especially in sheltered coves and bays. 

 It is a very active species and can swim rapidly ; it is therefore often 

 seen swimming at or near the surface. The full-grown individuals gen- 

 erally keep away from the shores, in shallow water, frequenting muddy 

 bottoms, especially among the eel-grass, and are also found in large 



