From this first theca originates a second, opening in the same direction, and from the second a third, and so on, in a continuous linear series until the polypary is complete.
Developed; it remains permanently attached to the dorsal wall of the polypary, of which it forms the proximal end, its apex rarely reaching beyond the third or fourth theca.
A fine cylindrical rod or fibre (the so-called solid axis or virgula) becomes developed in a median groove in the dorsal wall of the polypary, and is sometimes continued distally as a naked rod.
It was formerly supposed that a virgula was present in all the Graptoloidea; hence the term Rhabdophora sometimes employed for the Graptoloidea in general, and rhabdosome for the individual polypary; but while the virgula is present in many (Axonophora) it is absent as such in others (Axonolipa).
In all these families the polypary originates as in Monograptus from a nema-bearing sicula, which invariably opens downwards and gives off only a single bud, such branching as may take place occurring at subsequent stages in the growth of the polypary.
In some species young examples have been met with in which the nema ends above in a small membranous disk, which has been interpreted as an organ of attachment to the underside of floating bodies, probably sea weeds, from which the young polypary hung suspended.
In the oldest family - Dichograptidae--in which the branching polypary is bilaterally symmetrical and.
In the family of the Diplograptidae the branches are reduced to two; these also coalesce similarly by their dorsal walls, and the polypary thus becomes biserial (diprionidian), and the line of the nema is taken by a long axial tube-like structure, the nemacaulus or virgular tube.
The Monograptidae, the branches are theoretically reduced to one, the polypary is uniserial throughout, and all the thecae are directed outwards and upwards.
The thecae in the earliest family - Dichograptidae - are so similar in form to the sicula itself that the polypary has been compared to a colony of siculae; there is the greatest variation in shape in those of the latest family - Monograptidae--in some species of which the terminal portion of each theca becomes isolated (Rastrites) and in some coiled into a rounded lobe.
It is the general practice of palaeontologists to regard each graptolite polypary (rhabdosome) developed from a single sicula as an individual of the highest order.
In the Dendroidea, as a rule, the polypary is non-symmetrical in shape and tree-like or shrub-like in habit, with numerous branches irregularly disposed, and with a distinct stem-like or short basal portion ending below in root-like fibres or in a membranous disk or sheet of attachment.
An exception, however,, is constituted by the comprehensive genus Dictyonema, which embraces species composed of a large number of divergent and sub-parallel branches, united by transverse dissepiments into a symmetrical cone-like or funnel-shaped polypary, and includes some forms (Dictyograptus) which originate from a nema-bearing sicula and have been claimed as belonging to the Graptoloidea.
Of the early development of the polypary in the Dendroidea little is known, but the more mature stages have been fully worked out.
In the Graptoloidea certain lateral and vesicular appendages of the polypary in the Lasiograptidae have been looked upon as connected with the reproductive system; and in the umbrella-shaped synrhabdosomes already referred to, the common centre is surrounded by a ring of what have been regarded as ovarian capsules.