From this first theca originates a second, opening in the same direction, and from the second a third, and soon, in a continuous linear series until the polypary is complete. Each zooid buds from the one immediately preceding it in the series, and intercommunication is effected by all the budding orifices (including that in the wall of the sicula) remaining permanently open. The sicula itself ceases to grow soon after the earliest theca have been 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).
The Graptoloidea are arranged in eight families, each named after a characteristic genus: (1) Dichograptidae; (2) Leptograptidae; (3) Dicranograptidae; (4) Diplograptidae; (5) Glossograptidae (sub-family, Lasiograptidae); (6) Retiolitidae; (7) Dimorphograptidae; (8) Monograptidae.
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.
Broadly speaking, these families make their first appearance in time in the order given above, and show a progressive morphological evolution along certain special lines. There is a tendency for the branches to become reduced in number, and for the serial thecae to become directed more and more upwards towards the line of the nema. In the oldest family—Dichograptidae—in which the branching polypary is bilaterally symmetrical and the thecae uniserial (monoprionidian)—there is a gradation from earlier groups with many branches to later groups with only two; and from species in which all the branches and their thecae are directed downwards, through species in which the branches become bent back more and more outwards and upwards, until in some the terminal thecae open almost vertically. In the genus Phyllograptus the branches have become reduced to four and these coalesce by their dorsal walls along the line of the nema, and the sicula becomes embedded in the base of the polypary. 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. Finally, in the latest family, the Monograptidae, the branches are theoretically reduced to one, the polypary is uniserial throughout, and all the thecae are directed outwards and upwards.
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1, Diptograptus, young sicula. 2, Monograptus dubius, sicula and first serial theca (partly restored). 3, Young form (all above after Wiman). 4a, Older form. 4b, Showing virgula (after Holm). 5, Rastrites distans. 6, Base of Diptograptus (after Wiman). 7, D. calcaratus. 8, Dimorphograptus. 9, Base of Didymograptus minulus (after Holm). 10, Young Dictyograptus, with primary disk. 11, Ibid. Diptograptus (after Ruedemann). 12 a-b, Base and transverse section, Retiolites Geinitzianus (after Holm). 13, Bryograptus Kjerulfi. 14, Dichograptus octobrachiatus, with central disk. 15, Didymograptus Murchisoni. 16, D. gibberulus. 17 a-b, Phyllograptus and transverse section. 18, Nemagraptus gracilis. 19, Dicranograptus ramosus. 20, Climacograptus Scharenbergi. | 21, Glossograptus Hincksii. 22, Lasiograptus costatus (after Elles and Wood). 23, Dictyonema (-graptus) flabelliforme (-is). 24, Dictyonema (-dendron) peltatum with base of attachment. 25, D. cervicorne, branches (after Holm). 26, D. rarum (section after Wiman). 27, Dendrograptus Hallianus. 28, Synrhabdosome of Diptograptus (after Ruedemann). S, Sicula. u, Upper or apical portion. l, Lower or apertural. m, Mouth. N, Nema. nn, Nemacaulus or virgular tube. V, Virgula. vv, Virgella. zz, Septal strands. T, Theca. C, Common canal (in Retiolites). G, Gonangium. g, Gonotheca. b, Budding theca. |
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. The thecae in several of the families are occasionally provided with spines or lateral processes: the spines are especially conspicuous at the base in some biserial forms: in the Lasiograptidae the lateral processes originate a marginal meshwork surrounding the polypary.
Histologically, the perisarc or test in the Graptoloidea appears to be composed of three layers, a middle layer of variable structure, and an overlying and an underlying layer of remarkable tenuity. The central layer is usually thick and marked by lines of growth; but in Glossograptus and Lasiograptus it is thinned down to a fine membrane stretched upon a skeleton framework of lists and fibres, and in Retiolites this membrane is reduced to a delicate network. The groups typified by these three genera are sometimes referred to, collectively, as the Retioloidea, and the structure as retioloid.
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. Certain American forms, however, which are preserved as stellate groups, have been interpreted as complex umbrella-shaped colonial stocks, individuals of a still higher order (synrhabdosomes), composed of a number of biserial polyparies (each having a sicula at its outer extremity) attached by their nemacauli to a common centre of origin, which is provided with two disks, a swimming bladder and a ring of capsules.
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.