carapace and similar in appearance to the free somites. The genus Agnostus, which belongs to the last category, occurs abundantly in Cambrian strata and is one of the earliest forms known. This would lead to the supposition that the great development of metasomatic carapace is a primitive and not a late character, were it not for the fact that Paradoxides and Atops, with an inconspicuous telsonic carapace and numerous free somites, are also Cambrian in age, the latter indeed anterior in horizon to Agnostus.
On the other hand, it may well be doubted whether the pygidial or posterior carapace is primarily due to a fusion of the tergites of somites which were previously movable and well developed. The posterior carapace of the Trilobites and of Limulus is probably enough in origin a telsonic carapace—that is to say, is the tergum of the last segment of the body which carries the anus. From the front of this region new segments are produced in the first instance, and are added during growth to the existing series. This telson may enlarge, it may possibly even become internally and sternally developed as partially separate somites, and the tergum may remain without trace of somite formation, or, as appears to be the case in Limulus, the telson gives rise to a few well-marked somites (mesosoma and two others) and then enlarges without further trace of segmentation, whilst the chitinous integument which develops in increasing thickness on the terga as growth advances welds together the unsegmented telson and the somites in front of it, which were previously marked by separate tergal thickenings. It must always be remembered that we are liable (especially in the case of fossilized integuments) to attach an unwarranted interpretation to the mere discontinuity or continuity of the thickened plates of chitinous cuticle on the back of an Arthropod. These plates may fuse, and yet the somites to which they belong may remain distinct, and each have its pair of appendages well developed. On the other hand, an unusually large tergal plate, whether terminal or in the series, is not always due to fusion of the dorsal plates of once-separate somites, but is often a case of growth and enlargement of a single somite without formation of any trace of a new somite. For the literature of Trilobites see (22*).
|Fig. 35.—Triarthrus Becki, Green. a, Restored thoracic limbs in transverse section of the animal; b, section across a posterior somite; c, section across one of the|
|Fig. 36.—Triarthrus Becki, Green.
Dorsal view of second thoracic leg
with and without setae. en, Inner
ramus; ex, Outer ramus.
|Fig. 37.—Deiphon Forbesii,Barr. One of the|
Cheiruridae. Silurian Bohemia.
(From Zittel’s Palaeontology.)
|Fig. 38.—Dalmanites limulurus,
Green. One of the Phacopidae, from
the Silurian, New York.
|Fig. 39.—Megalaspis extenuatus.|
One of the Asaphidae allied to
Illaenus, from the Ordovician of
East Gothland, Sweden.
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, Scorpio), after embryonic appearance, or is
- Pocock suggests that the area marked vii. in the outline figure of the dorsal view of Limulus (fig. 7) may be the tergum of the suppressed prae-genital somite. Embryological evidence must settle whether this is so or not.