This is Jens’ work leading to the comparative investigations on free-living crinoids on Elba:
Jens Bohn (1997)
Computer-based reconstruction of the aboral nervous system of stalked crinoids
Diploma Thesis, Ludwig Maximilians University, Munich/ Germany, Institute for Zoology
The form of the aboral nervous system of the calyx of six different stalked crinoid species was reconstructed three-dimensionally via computer.
Pictures of slices of a transversal series were taken. Foils were laid over the pictures and the aboral nervous system was traced manually. This way it was isolated graphically from other structures of the organism. Furthermore, the drawings were scanned, aligned and reconstructed three-dimensionally on the computer.
By reconstruction of the aboral nervous system of the six observed species, two different types were found. The “Democrinus” type, typical for the five species belonging to the Bourgueticrinida (Bathycrinus, Democrinus, Rhizocrinus), differs in many ways from the “Isocrinus” type, which was found for the genus Melacrinus (Isocrinida).
In the “Democrinus” type the five radial nerves, coming from the arms, lead to a pentagonal nerve ring by which they communicate.
Towards aborally five nerve cords leave the ring inter-radially and lead to the stalk.
The “Isocrinus” type shows some differences. As mentioned for the “Democrinus” type, the five nerve cords meet in a pentagon. In this type though, five nerve-pairs run in aboral direction, not in an inter-radial position, but originating very closely to the radial plane. The nerves of each pair separate on their way to the aboral side. The space in between them increases until each nerve meets a nerve of the neighboured nerve-pair and five inter-radial nerves are formed. Between these five nerves the space gets smaller again; they converge and conjoin to form the aboral nervous centre.
On the aboral side, the five stalk nerves leave the centre in inter-radial position.
The comparison of the nervous systems of the “Isocrinus” type with the “Democrinus” type and the data known from literature leads to the following conclusions:
(1) The progression of the aboral nervous system in the calyx and in the stalk is a phylogenetic feature. Up to date, far too little attention has been given to this attribute, which could offer great contributions concerning the clarification of taxonomic relationships of the extant crinoids.
(2) All known types of an aboral nervous system (six up to date) can be derived from the “Isocrinus” type without any greater problems. This could implicate that they all evolved from this archetype.
(3) The “Democrinus” type is a synapomorhy of a part of the group of Bourgueticrinida. A modern system of the Bourgueticrinida should include them as a monophyletic group.