ii8 ANNELIDA OR ANNULATA 



is estimated that the average field soil has 150,000 earthworms to 

 the acre. 



Some fresh water Oligochaeta form tubes of mud held together by 

 a mucus secretion. Others like the marine Chaetopterus form a 

 yellow, parchment-like tube. Some species like Hydroides (^Serpidd) 

 form lime tubes on shells. Sabellaria, an aberrant form, builds 

 reefs on porous rocks from sandy tubes. Some species excavate 

 galleries in rock or corals. 



Behavior. — The palolo-worm^ Eunice, found in the Pacific coral 

 reefs, swarms during the last quarter of the moon in October and 

 November. The sexual posterior part of the worm (called the 

 epitoke) separates from the sexless anterior portion {atoke) and floats 

 on the sea, giving off spermatozoa and eggs. Fishermen prepare 

 their nets and boats and capture these worms in great numbers, 

 sometimes cooking them in leaves, but at other times eating them 

 alive. (See p. 102.) 



The Annelids are said to give little response to light and shadow 

 after they have become accustomed to them. But Copeland, 1930 

 (Jour. Comp. Psychol., vol. 10, p. 339), showed that in Nereis virens, 

 an apparent " conditioned response " was induced by either in- 

 creased or decreased illumination, which indicated to the worm the 

 presence of food. The earthworm draws leaves into the burrow to 

 line it. There is no exploration of the form of the leaf; it is seized 

 at any point, but only those seized at or near the apex get into the 

 burrow. 



Hirudinea. — The leeches {Hirudinea) are parasitic forms infest- 

 ing invertebrates as well as vertebrates. The majority of the leeches 

 live in fresh water and parasitize molluscs and the vertebrates. 

 Certain of them are permanent ectoparasites, Branchellion attaching 

 to various elasmobranchs. The giant leech {Macrobdella valvidi- 

 viana) may reach a length of i>^ feet and is subterranean and 

 carnivorous. Clepsine carries its young on the ventral surface. 

 The skate sucker {Pontobdella muricata) has a leathery knobbed skin. 

 It lays its soft eggs in empty mollusc shells and guards them for 

 over 100 days. Lophobdella lives on the lips and jaws of the Cro- 

 codilia. Certain intermediate types, the Myzostomata, parasitize 

 the feather stars, forming galls on them. 



Parasites of the Annelida. — Certain parasitic Nematodes, the 

 minute threadworms {Pelodera pellio), are found in the body cavity 

 and nephridia as well as the ventral blood vessels of the earthworm. 

 Various protozoa also infect the Annelida. 



