Lankester the arrangement is " nomomeristic " and " nomotagmic."
The most important general change which has affected the structure of the nomomeristic Arachnida in the course of their historic development is the transition from an aquatic to a terrestrial life.
The nomomeristic Arachnids comprise two sub-classes - one a very small degenerate offshoot from early ancestors; the other, the great bulk of the class.
- Nomomeristic Arachnids, in which the somites corresponding to mesosoma and metasoma have entirely aborted.
(of the Nomomeristic Arachnida).
They may be regarded as " macrourous " Xiphosura; that is to say, Xiphosura in which the nomomeristic number of eighteen From Zittel's Palaeontology.