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 ARACHNIDA 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. A

B

C

Fio. 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 prosomatic and telsonic carapaces ; D, adult condition, still with only two free mesosomatic somites. (From Korschelt and Heider.)

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*). Grade B {of the Arachnida) 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, (6) the “mesosoma” with plate-like appendages, and (c) the “metasoma” with suppressed appendages. A somite placed between the prosoma and mesosoma —the prre-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 either1 fused (with loss of its appendages) with the prosoma (Limulus, Scorpio), after embryonic appearance, or is retained as a rudimentary, separate, detached somite in front of the mesosoma, or disappears altogether (excalation). The atrophy Fig. 41.—Five stages in the development of the trilobite Sao Mrsuta. A, youngest stage;B, older stage with distinct pygidial carapace; C, stage with two free mesosomatic somites between the prosomatic and telsonic carapaces; D, stage with seven free intermediate somites ; E, stage 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. (From Korschelt and Heider, after Barrande.) 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 1 Mr 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 prse-genital somite. Embryological evidence must settle whether this is so or not.

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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 Aranese and Opiliones, and, on the other hand, may terminate in simplification and degeneration, as in the Acari. 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

Fig. 42.—So-called “trilobite stage” of Limulus polyphemus. A, dorsal, B, ventral view. (From Korschelt and Heider, after Leuckart.) 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 leg-bearing somite (the prse-genital rudimentary somite of Eu-arachnida) 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 Lsemodipoda. The genital pores are situate at the base of

Fig. 43.—One of the Nymphonomorphous Pantopoda, Nymphon Tiispidum, showing the seven pairs of appendages 1 to 7 ; ab, the rudimentary opisthosoma; s, the mouth - bearing proboscis. (From Parker and Haswell’s Text-book of Zoology, after Hoek.) 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 Aranese) and the genital products are developed in gonoccels also placed in the legs. The Pantopoda are divided into three orders, the characters of which are dependent on variation in the presence of the full number of legs. Order 1 (of the Pantopoda). Nymphonomorpha, Pocock (nov.) (Fig. 43).—In primitive forms belonging to the family Nymphonkke the full complement of appendages is retained—the 1st (mandibular), the 2nd (palpiform), and the 3rd (ovigerous) pairs being well developed in both sexes. In certain derivative forms constituting the family Pallenidoe, however, the appendages of the 2nd pair are either rudimentary or atrophied altogether. Two families: 1. Nymphonidse (genus Nymphon), and 2. Pallenidse (genus Pallene).