190 MARINE ANIMALS 



movement, so that they come to rest only where wave and currents are 

 absent. The deposition of mud occurs at various depths according to 

 local conditions. It accumulates in shallow waters on coasts protected 

 by strings of islands, in quiet bays, and at river mouths, where the 

 water stagnates twice daily on account of the opposing tidal current. 

 Mud flats in the Bay of Naples occur at 20 to 40 m., and at much less 

 depth in the Mergellina Harbor at Naples. In the open ocean, these 

 deposits do not come to rest until much greater depths are reached. 

 The "mud line," which marks the approximate limit of deposition of 

 muds from the land, lies on the average about 200 m. below the surface. 



The mud affords a rich food supply. Vast numbers of minute forms, 

 protozoans, nematodes and other worms, and swarms of small crusta- 

 ceans, ostracods, amphipods, and isopods, develop at the expense of the 

 organic food materials, and create a new source of food in their turn. 

 Mollusks bury themselves in the mud, and nudibranchs and crabs 

 wander over its surface.* 



The mud dwellers are on the whole more delicate than the sand 

 inhabitants: more fragile, thinner shelled, more weakly muscled. A 

 yellowish gray or white coloration is frequent among them. Degenera- 

 tion and loss of the eyes is common. These differences are excellently 

 illustrated by comparison of two sipunculid worms, Sipunculus nudus 

 from sand and Phascolosoma vulgare from the mud. In the mud dweller, 

 the skin is softer, the musculature weaker, motility less, nervous system 

 less well developed, and the proboscis longer and more slender. 12 



The mud dwellers have a mode of life in the main like that of the 

 sand bottom animals, but there are fewer of them, on account of the 

 want of oxygen in their habitat. The process of decay uses up the 

 oxygen supply, mainly by the oxidation of the hydrogen sulphide pro- 

 duced. Where the supply of fresh water is too small, the population 

 decreases with the decrease in oxygen and the increase in hydrogen 

 sulphide; and deep black, homogeneous mud, without admixture of 

 sand, is for the most part without microscopic life. Certain lamelli- 

 branchs and the tube worm Spio fuliginosus are the most persistent 

 survivors. Mixture of mud with sand is especially favorable, and pro- 

 duces an increase in the animal population. The mud line near the 



* According to J. Wilhelmi, u the index forms of the mud facies in the Bay of 

 Naples are Plagiostomum girardi (turbellarian) ; Audouinia, Arenicola, Capitella 

 capitata, Spio juliginosus, and Spirographis (annelids) ; Asterias tenuispina (star- 

 fish) ; Bornia corbuloides, Capsa fragilis, Tapes aureus (lamellibranchs) ; Bulla 

 striata and Doris verrucosa (opisthobranchs) ; Nebalia galatea and Brachynotus 

 sexdentatus (crustaceans) ; Botryllus aurolineatus and Ciona (tunicates) ; and 

 among fishes Mullus barbatus, Zeus faber, and Lophius piscatorius. 





