786 EMERY [chap. 28 



kinds of worms which live in the bottom sediments. Since most are deposit 

 feeders, they annually pass large quantities of sediment through their digestive 

 tracts, in the process making burrows and dumps of fecal pellets and destroying 

 the bedding of sediments. Such features are common in ancient as well as in 

 recent marine sediments (Schwartz, 1932; Shrock, 1935; Seilacher, 1953; 

 Schafer, 1956). 



As shown by Darwin (1883, pp. 159, 305), earthworms on average ground of 

 England bring up subsoil at a rate corresponding to a layer 0.5 cm thick each 

 year; or in other terms, the average of 6 earthworms (6 g) per square meter 

 moves about 2500 g of soil each year. The work of marine worms has not yet been 

 well measured, but biomasses commonly amount to more than 100 g/m^ in 

 bottom depths of less than 2000 m, suggesting that worms may even be more 

 important transporters of sediment on the sea floor than on land. It should be 

 noted, however, that most of this transport is vertical, rather than lateral, and 

 for only a few centimeters; thus, its main function is that of mixing sediment. 

 Outstanding is the work of the echiuroid worm, Echiurus echiurus, which lives in 

 U-shaped burrows about 30 cm deep in water depths of 10 to 30 m in western 

 Sweden (Gislen, 1940). Specimens measure about 10 cm in length and number 

 from 20 to 335 per square meter of bottom. A medium-sized specimen may ex- 

 crete daily a total of 540 pellets, each about 2 mm long. Similar intense activity 

 is probably characteristic of a larger echiuroid worm, Listriolobus pelodes, 

 which lives in such abundance in muds of the shelf off Santa Barbara, Cali- 

 fornia, as to constitute biomasses of 1100 grams per square meter (Barnard and 

 Hartman, 1959). 



During their passage between the hard tooth-like grinder stones in the worm's 

 gizzard, sand grains become somewhat rounded and worn, especially the softer 

 grains. Other modification of grain size of sediment occurs through the sorting 

 activities of some of the sand-reef building polychaetous worms, notably those 

 of the genera Phragmatopoma and Pectinaria. Species of Phragmatopoma are 

 known to select sand grains with such precision that the species can be identified 

 on the basis of grain size of sand composing their tubes. The form endemic to 

 Peru selects grains of the smallest size, the one in Ecuador takes medium sand, 

 and the ones in southern California and the West Indies use the coarsest sand 

 (Hartman, 1944). These species are not known from any other parts of the 

 world. Some terebellid worms select foraminiferal tests of particular kinds or 

 sponge spicules of a certain length and thickness; where these sj)ecies are 

 abundant, they can account for deposits of considerable extent. 



In shallow-water areas of coarse sand in the tropics holothurians probably 

 play a greater role than worms as a geological agent. Studies by Crozier (1918) 

 at Bermuda indicate that Stichopus mobii ingests 6 to 7 kg of sand per square 

 meter each year. During the passage of this sand through the intestinal tract it 

 becomes immersed in digestive fluids having a pH as low as 5.0. The resulting 

 solution of calcareous grains corresponds to about 400 g/year, according to 

 experiments by Mayor (1924) at Samoa; this amounts to aiDproximately one per 

 cent of all the sand which is ingested. Analyses by Emery (1962) of sand in and 



