GRAPTOLITES OF NEW YORK, PART 1 619 



an exactly analogous growth stage is produced [p.523j, the planula is dis- 

 charged, which does not produce disk, hydrocaulus and first theca till after 

 fixation. It is now inconceivable that these disks of the siculae of Diplo- 

 graptus could have been intended as organs of attachment, as, wherever 

 attachment of the embryo takes place the disk is formed only after attach- 

 ment and as a result of it. As the Axonolipa had changed from 

 pseiidoplanktonic to holoplanktonic forms, it is to be inferred that the disks 

 of the siculae originally were actually organs of attachment, but assumed 

 secondarily the function of floats, and that then, by acceleration of develop- 

 ment or retention and brooding of the embryos within the gonothecae, the 

 disks, which originally formed only after contact of the embryos with foreign 

 bodies, were produced within the gonangia. 



It would finally appear that the change from a pseudoplanktonic 

 mode of existence by attachment to seaweeds to a holoplanktonic mode 

 would be easily accomplished by the accumulation of gas in some portion 

 of the supporting organs. It would, for instance, be possible that a 

 transudation of gas took place into the space between the principal layer 

 of the perisarc and the epidermal layer of the primary disk, thus pro- 

 ducing the tenuous " pneumatophor " described by the writer. Further, 

 the vesiclelike expansions of the nemacaulus and of the distal appendages 

 of certain species of Diplograptus, as I), physophora, vesiculosus 

 and laxus [pi. 16, fig.3] are plausibly explained as organs which assisted 

 in floating the synrhabdosomes, and eventually protected the broken-off 

 rhabdosomes from sinking. 



7 Mode of reproduction and ontogeny 



Early growth stages of graptolites are of very common occurrence in 

 the graptolite beds. They, therefore, soon attracted the attention of 

 observers; and Hall has figured [1865, pl.B] a number of such "germs" 

 without, however, referring them to any definite species. He also recognized 

 clearly that the growth of the rhabdosqme begins with the minute spinelike 



