BIONOMIC CHARACTERS OF COILENTERATA loii 



and they usually occur in the more moderate depths. The tubularian 

 hydroids probably never extend to any considerable depths, the 

 deep-water forms belonging chiefly to the Plumularidse. (Wgassiz- 

 I, ii:j5.) One of the abyssal Phmuilarians was obtained by the 

 Blake at a depth of 1,240 fathoms, which exceeded by more than 

 300 fathoms that at which Plumularians were obtained by the Chal- 

 lenger. (Agassiz-i.) 



The Palaeozoic class of graptolites or Graptozoa is the most im- 

 portant group of the Coelenterata from a stratigrapher's point of 

 view, for it constitutes one of the most important classes of index 

 fossils known. 



Lapworth ( \\^alther-29 ) holds that the majority of dendroid 

 graptolites (Dendroidea) undoubtedly grew attached to sea-weeds, 

 rocks, or other supports, in the manner of most modern hydroids, 

 but some were attached to floating algse, leading an epi-planktonic 

 existence. Cases of such attachment have been observed among 

 these fossils. 



Lapworth argues that, if the sicula was attached by means of 

 the slender basal thread, the nema, to floating objects of support, 

 whether disc or sea-weed, the second and succeeding hydrothecje, 

 growing in the same direction as the sicula, would open downward. 

 This suggests that the earlier graptolites were not planktonic, but 

 grew attached to sea- weeds and rocks after the manner of modern 

 hydroids. This view is taken by Hahn (16) with reference to 

 Dictyonema. In later genera of graptolites, however, which may 

 have been attached to floating objects (epi-plankton), the branches 

 either bent backward, so as to cause the later hydrothecje to open 

 in an opposite direction from the sicula and early hydrothecse ; or 

 the direction of growth was reversed, the second and succeeding 

 hydrothecse growing backward along the nema, which became the 

 supporting rod or virgula. 



Some of the graptolites appear to have led a holoplanktonic ex- 

 istence, the nema being attached to a central organ or disc, which 

 probably served as a float. This was long ago demonstrated in a 

 number of species by Professor Hall, and lately has been showai in 

 great detail in Diplograptus by Ruedemann. This observer holds 

 that this mode of attachment was characteristic of the virgulate 

 graptolites (Axonophora) as a whole, while the AxonoUpa he 

 thinks were attached to seaweeds. (Ruedemann-25 :5if).) Whether 

 holo-planktonic or epi-planktonic, either method of life accounts 

 for the wide distribution of the graptolites. The fact that they are 

 almost universally found in carbonaceous shales suggests that float- 

 ing algae may have been the principal carriers of these organisms. 



