The organisms included at present under this head are all extinct, and they are in many respects so dissimilar, and their structure is so far from being entirely understood, that it is doubtful if any definition can be framed which will include all the supposed members of the family. The following definition, however, will include all the most typical Graptolites:
Hydrosoma compound, occasionally branched, consisting of numerous polypites united by a coenosarc; the latter being enclosed in a strong tubular polypary, whilst the former were protected by hydrothecae. In the great majority of Graptolites the hydrosoma was certainly unattached; but in some aberrant forms - doubtfully belonging to the sub-class - there is reason to believe that the hydrosoma was fixed. The polypites are never separated from the coenosarc by any partition. In many cases the hydrosoma was strengthened by a chitinous rod, the "solid axis," somewhat analogous to the chitinous rod recently described by Professor Allman in the singular Poly-zoon Rhabdopleura.
From the above definition it will be seen that the nearest living allies to the Graptolites are the Sertularians. In point of fact, if we do not insist upon the presence of a "solid axis" as part of the definition, the Graptolites differ from the Sertularians in no essential point, save that the hydrosoma is always attached in the latter, and was certainly free in the most typical examples of the former. Indeed, certain forms at present placed among the Graptolites - such as Ptilograptus and Dendrograptus - are so similar to some living Sertularians, that it might be well to remove them altogether from the Graptolitidae, and to regard them as extinct representatives of the Sertularida.
As regards the value of the "solid axis" as an element in defining Graptolites, it is doubtful if much stress can be laid upon its presence or absence. It is true that it is present in all the most characteristic members of the sub-class, but it has not been proved to be present in some forms, which in other respects are quite typical members of the group.
Taking such a simple Graptolite as G. priodon (fig. 62) as the type of the sub-class, the hydrosoma is found to consist of the "solid axis," the "common canal," and the "cellules." The entire poly-pary is corneous and flexible, and the solid axis is a cylindrical fibrous rod, which gives support to the entire organism, and is often prolonged beyond one or both ends of the hydrosoma. There is, however, every reason for believing that the so-called "solid axis" is truly hollow. The common canal is a tube which encloses the coenosarc, and gives origin to a series of cellules, these being little cups corresponding to "hy-drothecae," and enclosing the polypites. Not only are the essential details of the structure - with the exception of the solid axis - strictly comparable with that of a Sertularian, but there is good evidence that the reproductive process, in some forms at any rate, was also carried on in a manner similar to what we have seen in some other Hydroida - namely, by generative buds or gono-phores enclosed in gonangia.
No Graptolite, however, has hitherto been certainly proved to have been fixed by a "hydrorhiza," and it is only in certain aberrant forms that there are any traces of a "hydro-caulus."
Besides the simple forms of Graptolites with a row of cellules on one side (monoprionidian) (fig. 62), there are others with a row of cellules on each side (diprionidian) (fig. 63). Many other modifications are known ; but there is only another peculiarity which is worthy of notice here. This is the occurrence in several genera of a basal corneous disc or cup, which is probably the homologue of the "float "or "pneumatophore" of the Phy-sophoridae. (For distribution of Graptolites see Distribution of Hydrozoa in Time.
As regards their mode of occurrence, Graptolites are usually found as glistening, pyritous impressions, with a silvery lustre. In some cases, however, they are found in relief.