ontology.)]

[Illustration: FIG. 38.--_Dalmanites Kmulurus_, Green. One of the
_Phacopidae_, from the Silurian, New York.

(From Zittel.)]

[Illustration: FIG. 39.--_Megalaspis extenuatus_. One of the
_Asaphidae_ allied to _Illaenus_, from the Ordovician of East
Gothland, Sweden.

(From Zittel.)]

_Grade B (of the Aracknida) NOMOMERISTICA._--Arachnida in which,
excluding from consideration the eye-bearing prosthomere, the somites
are primarily (that is to say, in the common ancestor of the grade)
grouped in three regions of six--(a) the "prosoma" with palpiform
appendages, (b) the "mesosoma" with plate-like appendages, and (c) the
"metasoma" with suppressed appendages. A somite placed between the
prosoma and mesosoma --the prae-genital somite--appears to have
belonged originally to the prosomatic series (which with its ocular
prosthomere and palpiform limbs [Pantopoda], would thus consist of
eight somites), but to have been gradually reduced. In living
Arachnids, excepting the Pantopoda, it is either fused (with loss of
its appendages) with the prosoma (_Limulus_,[7] _Scorpio_), after
embryonic appearance, or is retained as a rudimentary, separate,
detached somite in front of the mesosoma, or disappears altogether
(excalation). The atrophy and total disappearance of ancestrally
well-marked somites frequently take place (as in all Arthropoda) at
the posterior extremity of the body, whilst excalation of somites may
occur at the constricted areas which often separate adjacent
"regions," though there are very few instances in which it has been
recognized. Concentration of the organ-systems by fusion of
neighbouring regions (prosoma, mesosoma, metasoma), previously
distinct, has frequently occurred, together with obliteration of the
muscular and chitinous structures indicative of distinct somites. This
concentration and obliteration of somites, often accompanied by
dislocation of important segmental structures (such as appendages and
nerve-ganglia), may lead to highly developed specialization
(individuation, H. Spencer), as in the Araneae and Opiliones, and, on
the other hand, may terminate in simplification and degeneration, as
in the Acari.

[Illustration: FIG. 40.--Four stages in the development of the
trilobite _Agnostus nudus_. A, Youngest stage with no mesosomatic
somites; B and C, stages with two mesosomatic somites between the
pr