I024 PRINCIPLES OF STRATIGRAPHY 



Besides calcareous tubes, many worms build tubes of aggluti- 

 nated sand grains or shell particles, and worms living in the shells 

 of dead molluscs 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 brings large 

 numbers of these tubes, together with numerous castings, from 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 characteristic, specimens having been ob- 

 tained from a depth of 4,000 fathoms off Teneriffe (Challenger). 

 Other worms also occur. Even species of the same genus have a 

 widely varying distribution in depth. Thus the tubicoloid genus 

 Spirorbis has its littoral species growing within the shore zone ; 

 while another species, S. nautiloides, has been dredged at a depth 

 of 700 fathoms. Similarly, the Sipunculid Phascolosoma is repre- 

 sented along our northern shores by a species living in mud and 

 sand above low tide, while the Blake brought up a species in a Den- 

 talium shell from a depth of 1,568 fathoms. (Agassiz-i, ii:55.) 

 The Myzostomidse are parasitic on living Comatula, and also have 

 been found on the column segments of Jurassic crinoids (Graff). 



Among the annelids, the family Eunicidae is of particular in- 

 terest, in that its several members are characteristic of diff'erent 

 bathymetric zones, thus furnishing, in a measure, an index to the 

 bathymetric position of the fauna which they characterize. This 

 family is well represented in the lithographic shales of Bavaria. 

 (Ehlers.) 



Among the worms regeneration of lost parts and generation of 

 new individuals from fragments of old ones are not uncommon. 

 Thus, in one of our common halo-pelagic worms, Autolytus, swim- 

 ming buds carrying the sexual products are periodically constricted 

 off, each regenerating a new head, with highly developed eyes at 

 the anterior end. The oligochste Lumbriculus in autumn falls 

 into pieces, all of which are able to regenerate into complete ani- 

 mals. (Lang-19; \:267.) 



A mero-planktonic ciliated larva, the trochophore, is character- 

 istic of worms, this being the product of a sexual mode of repro- 

 duction. These larvse are often obtained in vast numbers in the 

 ,tow-net, together with other mero-planktonic and many holo-plank- 

 tonic forms. 



The CESophageal teeth of annelids are abundantly represented 

 in many bituminous shales, from the Palaeozoic on. Some of these 



