zoology: marine invertebrates. 95 



forbesii is abundant and varies greatly in color. A. vulgaris is 

 also common. The smooth star-fish, Cribrella sanguineolenta , with 

 only two rows of ambulacra in each ray, not infrequently is found, 

 although on the whole it is a rare species on our coast. 



The brittle star, Ophiopholis aculeata, is common in the pools, 

 hiding away under stones and in crevices or clinging to the rocks 

 under the seaweed. This species is much more common in the 

 deeper waters off shore, where the dredge brings it up by the 

 hundred. The common sea urchin of the north Atlantic coast, 

 Strongylocentrotus drobachiensis , is very abundant in the Nahant 

 tide-pools. It may be found always in the deeper pools, hiding 

 away in the corners and covering itself with shells, seaweeds, and 

 other available protective coverings. This species is exceedingly 

 abundant on some parts of the Maine coast, where it is uncovered 

 by the thousand at low tide. It rests then among the stones, 

 and is always more or less covered by dead shells, pebbles, etc. 



Crustacea are not uncommon in the tide-pools, where several 

 species of decapods always may be met. After these and the bar- 

 nacles, the most prominent forms are the isopods Idotea irrorata 

 and I. phosphorea. The latter is found not infrequently swim- 

 ming in numbers in these pools. Both species appear to be more 

 abundant farther north, on the coast of Maine, where they are 

 among the principal inhabitants of the tide-pools on exposed 

 shores. The two species are distinguished readily by the form of 

 the pleon, which in /. irrorata has a tridentate termination, while 

 in /. phosphom i. it is pointed. Both forms are large, an inch or 

 more in length, and both vary much in color, the chief tints being- 

 greenish and brownish. /. irrorata is commonly striped, while 

 /. phosphorea is more often banded. 



In addition to the animals mentioned from these tide-pools, a 

 large number of others occur, including probably many unde- 

 scribed species ; thus furnishing an ideal spot for the student of 

 marine invertebrates. 



GENERAL REFERENCE WORKS. 



1851. Stimpson, milium. Shells of New England. 



1853. Stimpson, William. Marine Invertebrata of Grand Manan. 



(Smithsonian contributions to knowledge.) 

 1862. Verrill, A. E. Revision of the Polypi of the eastern coast of the 



United States. (Bost. Soc. Nat. Hist., Mem., vol. 1, pp. 1-45, 



pi. 1.) 



