THE PLUMULARID.iE. 29 



Allman has come to the conclusion that the nematophores of the Plumuktrida-. are homologous 

 with the denticles of graptolites. Not having had an opportunity to study these structures with 

 care, I here give Allman's argument, or the most important part of it, in his own words: 



The remarkable bodies known as nematopbores, and which are characteristic of the Plumnlaridic, have been 

 already described. Among these nematophorcs there is one form which consists of simple chitiuous offsets from the 

 main tube of the hydroid filled with the protoplasmic matter which constitutes the characteristic contents of all the 

 neinatophorcH. The mesial and lateral nematophores of Ai/laophenia are of this nature, and a comparison of them 

 with the denticles of a graptolite will show how complete is the resemblance. * It has been already shown 



that the toothliko processes which project from the edges of the hollow leaflets, which form the walls of the eorbnla 

 in Aglaophenia, are bodies of an entirely similar kind, and the resemblance between these and the toothlike processes 

 of many graptolites is complete. 



Now, it is not alone in general form that the nematophores of Ai/laojilienid resemble the denticles of a graptolite. 

 The mode in whicli their chitinous sheaths are seen to open into the common canal of Ihe perisarc after the destruction 

 of all the soft parts, is entirely similar to the mode of communication between the denticles and the common canal in 

 the fossil in those cases at least in which the graptolite lias afforded facilities for examination such as to leave, no 

 doubt as to the structure of the parts in question and quite different from that in which the proximal extremity of 

 the hydrotheca is connected with the common tube of the chitinous perisarc in the existing hydroid. 



I can not help believing that this is the true view to take of the' morphology of graptolites. If so, the grapto- 

 lites would admit of an approximation through an unexpected channel with thePlumularida;. They would then be 

 morphologically pluniularidaus in which the development of hydrothecie had been suppressed by the great devel- 

 opment of the nematophores, probably the mesial ones ; while, on the other hand, the existing pliimnlaridan, with 

 well-developed hydrotheca', would present in its nematophores the last traces of the structure of its ancient 

 representative, the graptolite. 1 



The idea that the nematophores and not the hydrotheca^ are homologues of the denticles of 

 the graptolites receives considerable support from the fact that the nematophores are, judging from 

 embryological evidence, older structures than the hydrotheca-, being developed in most, if not all, 

 cases before them in the history of the individual colony. Although this evidence is by no means 

 conclusive, it at least suggests that the hydranth is more modern than the sarcostyle. Almost 

 nothing seems to be known of the living parts of the graptolites, and it is hardly possible to come 

 to any definite conclusion regarding the relationships of these forms which have apparently by 

 common consent been included among the hydroids by several modern writers, notably von 

 Lendeufeld,- 2 who places them between the sertularians and plumularians. 



GONOSOME. 



This term, originally introduced by Alluiau, is used to designate all those, parts of a colony 

 which are concerned directly in the reproductive process (that is, the gouophores and their con- 

 tents), and also those structures destined for the protection of the gonophores. In the latter cate- 

 gory would come the goiiaugia, corbuhe, and phylactocarps of every description, from the most 

 complicated structures found in Lytocarpun and Aglaophenia to the simple protective branchlets 

 of certain species of Cladocarpus. 



GonopUores.Tkv gouophores of the Plumularidas are without exception protected by gonaugia. 

 When reduced to their fundamental plan they are simple hernialike protuberances from the 

 blastostyle, which are made up of the two histological layers, the endoderm and the ectoderm, the 

 two layers being separated by a thin, apparently structureless membrane, the "stutzlamelle" of 

 German writers (fig. 03, xt). The generative elements, when mature, are found between the stutzla- 

 melle and the ectoderm. The endoderm, with its included axial cavity, forms the spadix, which is 

 surrounded by a mass of spermatozoa in the male, and is pressed to one side by the developing ova 



in the female. 



The blastostyle is itself a diverticulum from the eu'iiosarc of the hydrocaulus, from which it 

 arises in the same way that the gouophore arises from the blastostyle. The distal or upper part 

 of the blastostyle in many calypteroblastic forms, including the Plumularida-, is developed during 

 the growth of the sperm masses or ova into a structure which acts as a thick plug in the end of the 

 gonangium, composed largely of ectoderm cells and called ' Deckenplatte" by Weismaim (tig. 93, ) 



Allman regards the blastostyle as a modified hydrauth, and there is little doubt of the correct- 

 ness of this view, which seems to be demonstrated in the case of certain gymoblastic forms (for 

 example, Eudentlrium) in which the hydranth loses its character as a hydranth, the tentacles become 



1 A Monograph of the Gymuoblastic or Tubularian Hydroids, Kay Society, London, 1871, p. 179. 



-Ueber Ca-lenteraten der Siidsee, V, Mittheilung. Zeitschrift fur wissenschaftliche Zoologie, XLI, p. Ulil. 



