Development axd Mode of Growth of Diplograptus. 231 



important fact is that the vesicles contained the so-called " siculae." 

 Since J. Hall discovered these tiny fossils and demonstrated that 

 they are the initial points of the growth of the rhabdosomes, 

 there has been no doubt that they represent a very young growth- 

 stage. It is, therefore, obvious that the described vesicles are 

 reproductive organs. 



It remains now for us to see how they compare with the repro- 

 ductive organs of the Sertularians, which have been regarded as the 

 living relatives of the Graptolites. - A glance at the reproductive 

 organs of the HydroZ'.^a is sufficient to demonstrate the great 

 similarity between them and those of the two Diplograptidae. 

 The gonophores and sporosacs of the Hydrozoa appear in just 

 such verticils of spherical or oval vesicles as the gonosome of the 

 two Graptolites, and they contain in the " spadix " an organ simi- 

 lar to the central vesicles, from which the siculae radiate. But 

 the gonophores, which directl}^ produce the generative elements, 

 are only covered by a thin pellicle, and are found only in those 

 Hydrozoans which have a thin, not chitinous perisarc ; whereas 

 the Sertularians, like all those Hydrozoans which are provided 

 with protective receptacles for the hydranths, inclose their sexual 

 buds, the gonophores or sporosacs, again in peculiar horny recep- 

 tacles, the gonangia. The latter are mostly oval capsules, formed 

 by a layer of ectoderm which secretes an external chitinous 

 investment, that varies greatly in thickness. " In every instance," 

 says Allmann, " where a gonangium exists the hydranths also are 

 protected by a hydrotheca, while the absence of a gonangium is 

 always associated with the absence of a hydrotheca." As we 

 have in our Graptolites, just as in the Sertularians^ chitinous 

 thecae and also a chitinous gonosome, we must term their 

 reproductive organs " gonangia." The gonangium of the Sertu- 

 larians contains a cylindrical column, the " blastostyle," bearing 

 the gonophores as buds upon its sides, and being generally 

 expanded at its summit into a conical plag or disc by which the 

 gonangium is closed. "We have a very similar organ in the club- 

 shaped hollow central vesicle of the gonangia of Dijplograjptus* 



* Cf . George J. Allmann: A Monograph of the Symnoblastidae or Tubular Hydroids. Vols. J and 

 II., 1872. 



G. J. Allmann: Report on the Hydroidea of the Gulf Stream. Museum of Comp. Zoology, Harvard 

 College. Vol. V, 188T7. 



Challenger Reports. Hydroidea, by Prof. G. J. Allmann. 



