LIFE IN THE OCEAN CLARK 387 



starfishes and the comatulids can swim freely for short distances; 

 some, like oysters and most stalked crinoids, live permanently at- 

 tached to other objects. 



To mention in detail the economic interrelationships of all these 

 creatures would be an overwhelming task ; and, indeed, very little is 

 known about them. But let us consider one or two examples. 



The common mussel, like oysters and other bivalves, is preyed upon 

 by the common starfish, which is often most terribly destructive, 

 moving back and forth across the mussel beds in swarms and up and 

 down the piles where mussels grow. We have no figures on the dam- 

 age done the mussels, but in 1888, on the Connecticut coast alone, this 

 starfish destroyed $631,500 worth of oysters, after not less than 

 42,000 bushels of them had been taken from the beds. Mussels are 

 preferred to oysters by the starfish, and some beds have been entirely 

 destroyed by them. Various gastropods, oysterdrills, dogwelks, 

 winkles, conchs, and others eat vast quantities, while killifish, cunners, 

 scup, tautog, squeteague, flounders, and cod are very fond of them. 

 In fact, mussels are excellent bait for fish. The walrus in the Green- 

 land seas feeds exclusively on mussels, though the seals, like dolphins, 

 feed on fish and squid. On our coasts mussels are eaten by rats and 

 by such birds as large gulls, ravens, crows, and ducks. 



Within the shells of oysters, pinnas, and the other larger bivalves 

 live flatworms, known as "wafers," little crabs, small shrimps, and 

 sometimes other things, most of which, except the first, are harm- 

 less, or at least do no more than steal the food that they collect. 



The very bony crinoids would seem to offer little in the way of 

 food for parasites, yet nearly 150 parasitic or semiparasitic forms 

 have been described from them. A little groove runs down the 

 middle of the upper side of the pinnules and the arms .of crinoids, 

 and the five grooves from the five rays converge to the central 

 mouth. The minute creatures taken from the water are passed 

 down along these grooves in a constant stream, which becomes 

 richer and richer as more and more of the victims are delivered 

 to it by the side branches, and at the mouth forms a rich plankton 

 soup. Most of the crinoids' parasites are simply grafters, camp- 

 ing along the sides of this stream and sucking up the soup. About 

 two-thirds of these belong to a curious type of worm, called ray- 

 zostomes, which, except for three sorts, internal parasites in star- 

 fishes or brittle stars, are entirely confined to them. Crinoids sup- 

 port about two dozen kinds of crustaceans of several different types, 

 a few of which bore into the soft parts, but most of which ap- 

 propriate the food material they collect, either from their ambulacral 

 grooves or from their stomachs. Nearly a dozen kinds of brittle 

 stars have never been found except upon them, about a dozen kinds 

 of small gastropods bore into them and suck their juices, and they 

 support at least one internal worm and many protozoans. Barnacles, 



