Encyclopaedia Britannica, 11th Edition, "Apollodorus" to "Aral" / Volume 2, Slice 3

by Various

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 sub-terminal somites.

(After Beecher.)

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.
(After Beecher.)	(From Zittel’s Palaeontology.)

Fig. 38.—Dalmanites Kmulurus, 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.
(From Zittel.)	(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.

	Fig. 40.—Four stages inthe development of thetrilobite Agnostus nudus.A, Youngest stage withno mesosomatic somites;B and C, stages with twomesosomatic somites betweenthe prosomatic andtelsonic carapaces; D,adult condition, still withonly two free mesosomaticsomites.
	(From Korschelt and Heider.)

	From Korschelt and Heider, after Barrande.
	Fig. 41.—Five Stages in the development of the trilobite Sao hirsuta.
	A, Youngest stage. B, Older stage with distinct pygidial carapace. C, Stage with two free mesosomatic somites between the prosomatic and telsonic carapaces. D, Stace with seven free intermediate somites. E, Stave with twelve free somites; the telsonic carapace has not increased in size. a, Lateral eye. g, So-called facial “suture” (not really a suture). p, Telsonic carapace.

	Fig. 42.—So-called“trilobite stage” ofLimulus polyphemus.A, Dorsal; B, ventralview.
	(from Korschelt and Heider,after Leuckart.)

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. This has been accompanied by the conversion of the lamelliform gill-plates into lamelliform lung-plates, and later the development from the lung-chambers, and at independent sites, of tracheae or air-tubes (by adaptation of the vasifactive tissue of the blood-vessels) similar to those independently developed in Peripatus, Diplopoda, Hexapoda and Chilopoda. Probably tracheae have developed independently by the same process in several groups of tracheate Arachnids. The nomomeristic Arachnids comprise two sub-classes—one a very small degenerate offshoot from early ancestors; the other, the great bulk of the class.

Sub-Class I. (of the Nomomeristica). PANTOPODA.—Nomomeristic Arachnids, in which the somites corresponding to mesosoma and metasoma have entirely aborted. The seventh, and sometimes the eighth, leg-bearing somite is present and has its leg-like appendages fully developed. Monomeniscous eyes with a double (really triple) cell-layer formed by invagination, as in the Eu-arachnida, are present The Pantopoda stand in the same relation to Limulus and Scorpio that Cyamus holds to the thoracostracous Crustacea. The reduction of the organism to seven leg-bearing somites, of which the first pair, as in so many Eu-arachnida, are chelate, is a form of degeneration connected with a peculiar quasi-parasitic habit resembling that of the crustacean Laemodipoda. The genital pores are situate at the base of the 7th pair of limbs, and may be repeated on the 4th, 5th, and 6th. In all known Pantopoda the size of the body is quite minute as compared with that of the limbs: the alimentary canal sends a long caecum into each leg (cf. the Araneae) and the genital products are developed in gonocoels also placed in the legs.

From Parker and Harwell’s Text-book of Zoology, after Hoek.

Fig. 43.—One of the Nymphonomorphous Pantopoda, Nymphon hispidum, showing the seven pairs of appendages 1 to 7; ab, the rudimentary opisthosoma; s, the mouth-bearing proboscis.