222 HYDROZOA. 



groups) is especially characterised by the presence of such genera as 

 Dichograptus and Tetragraptus, species of these genera being found 

 in rocks of this age in localities as remote as Canada, Britain, and 

 Australia. 



Finally, as regards the zoological affinities of the Graptolites, 

 palaeontologists are now agreed in regarding these singular organ- 

 isms as an ancient and aberrant type of the Hydrozoa. In many 

 respects the Graptolites present a resemblance to the recent Sertu- 

 larians, this resemblance being shown not only in the fact that the 

 hydrothecae have a general likeness in form and arrangement in the 

 two groups, but also in the possession by certain of the former of 

 chitinous reproductive capsules, which admit of comparison with 

 the " gonangia " of the latter. On the other hand, the polypary of 

 the Sertularians is always fixed, the hydrothecse are never in contact, 

 and the colony is not specially strengthened by a horny fibre. In 

 contradistinction, the Graptolites were free organisms, originating in 

 a basal " sicula," the hydrothecae are usually (but not always) more 

 or less largely in contact, and the polypary is strengthened by the 

 peculiar chitinous rod which has been previously spoken of as the 

 " virgula." In the singular recent genus Rhabdopleura, one of the 

 marine Polyzoa, the polyzoary is strengthened by a peculiar hollow 

 chitinous axial tube, which has been compared by Professor All- 

 man with the "virgula" of the Graptolites. There are, also, points 

 of resemblance between the Graptolites and those Polyzoa (such as 

 Vesicularia) in which the cells of the colony communicate by means 

 of a common tube. None of the Polyzoa, which possess a chitinous 

 external skeleton, are, however, free and unattached, and the general 

 balance of evidence is unquestionably in favour of a reference of the 

 Graptolitoidea to the class of the Hydrozoa. 



