348 



these animals, though many Hve among coarse blocks, and 

 vast numbers occnr among the coral masses of every coral 

 reef. The ciliated larva, or Anricularia, of the Holothnrians 

 is a mero-planktonic organism, with definite mouth and 

 anal opening. 



From the fact that only isolated plates occur in the skin 

 of the Holothnrians, they do not constitute any important 

 part of marine deposits. 



Nemathelminthes, Gephyrea, Annelida. — These worms are 

 marine, fresh-water, or terrestrial animals, with the exception 

 of the (iephyrea, which are wholly marine. They belong chiefly 

 to the benthos, though some marine forms lead a partially 

 nektonic existence, while others are typically planktonic. 

 Among the benthonic species all grades of a sedentary life 

 are observable, from the tube-building orders, which live 

 permanently in attached tubes, to those which only tem- 

 porarily occupy a given area. Tubicolar worms, which, like 

 Spii'orbis, attach their tubes chiefly to algte, nmy often lead 

 a pseudo-planktonic existence when such algae are torn from 

 their anchoi-age and drifted away by currents. 



Muddy bottom seems to be the favorite haunt of the 

 littoral species, except such forms as build attached tubes 

 (e. g., Serpulidte, etc.), which occupy stony and shelly bot- 

 toms. These latter often build extensile reefs of interwoven 

 calcareous tubes. 



Besides calcareous tubes, many worms build tubes of 

 agglutinated sand grains or shell particles, and \\'orms liv- 

 ing in the shells of dead mollusca are frequently met with. 

 These agglutinated sand tubes are often very resistant, 

 sometimes, with the castings, covering the mud flats and 

 beaches in great numbers, and not infrequently being heaped 

 together in windrows. The dredge often brings large num- 

 bers of these tubes, together with numerous castings, fi*om 

 the deeper water. The bathymetric distribution of the 

 worms is varied. The majority are undoubtedly littoral 

 species, but deep-sea forms are also common. Beyond the 

 hundred fathom line, the tubicolar Annelids are the most 



