Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/583

 ARACHNIDA tion of the limbs in Phyllopods inconsistent with their relationship to the Arthrostracous Crustaceans such as Gammarus and Oniscus. Thus, then, it seems that we have in the Trilobites the representatives of the lower phases of the Arachnidan pedigree. The simple Anomomeristic Trilobite, with its equi-formal somites and equi-formal appendages, is one term of the series which ends in the even more simple but degenerate Acari. Between the two and at the highest point of the arc, so far as morphological differentiation is concerned, stands the Scorpion ; near to it in the Trilobite’s direction (that is on the ascending side) are Limulus and the Eurypterines—with a long gap, due to obliteration of the record, separating them from the Trilobite. On the other side—tending downwards from the Scorpion towards the Acari — are the Pedipalpi, the Spiders, the Book - Scorpions, the Harvest-men, and the Water-Mites. The strange Nobody - Crabs or Pycnogonids occupy a place on the ascending half of the arc below the Eurypterines and Limulus. They are strangely modified and degenerate, but seem to be (as explained in the systematic review) the remnant of an Arachnidan group holding the same relation to the Scorpions which the Lsemodipoda hold to the Podophthalmate Crustacea. We have now to offer a classification of the Arachnida and to pass in review the larger groups, with a brief statement of their structural characteristics. In the bibliography at the close of this article (referred to by leaded arable numerals in brackets throughout these pages), the titles of works are given which contain detailed information as to the genera and species of each order or sub-order, their geographical distribution and their habits and economy so far as they have been ascertained. The limits of space do not permit of a fuller treatment of those matters here. Tabulae Classification1 of the Arachnida. Class. ARACHNID A. Grade A. ANOMOMERISTIC A. Sub-Class. TRILOBITiE. Orders. Not satisfactorily determined. Grade B. NOMOMERIST1CA. Sub-Class I. PANTOPODA. Order 1. Nymphonomorpha. ,, 2. Ascorhynchomorpha. ,, 3. Pycnogonomorpha. Sub-Class II. EU-ARACHNIDA. Grade a. delobkanchia, Lankester {vel hydropnetjstea, Pocock). Order 1. Xiphosura. „ 2. Gigantostraca. Grade 6. embolobranchia, Lankester {vel aeropnetjstea, Pocock). Section a. Pedinifera. Order 1. Scorpionidea. Sub-order a. Apoxypoda. „ b. Dionychopoda. 1 The writer is indebted to Mr R. I. Pocock, assistant in the Natural History departments of the British Museum, for valuable assistance in the preparation of this article and for the classification and definition of the groups of Eu-aracimida here given. The general scheme and some of the details have been brought by the writer into agreement with the views maintained in this article. Mr Pocock accepts those views in all essential points and has, as a special student of the Arachnida, given to them valuable expansion and confirmation. The writer also desires to express his thanks to Messrs. Macmillan for permission to use Figs. 22, 43, 44, and 45, which are taken from Parker and Haswell’s Text-book of Zoology; and to Messrs. Swan Sonnenschein and Co. for the loan of several figures from the translations published by them of the admirable treatise on Embryology by Professors Korschelt and Heider ; also to the publishers of the treatise on Palaeontology by Professor Zittel, Herr Oldenbourg and Macmillan and Co. of New York for several cuts of extinct forms.

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Section /3. Epedinata. Order 2. Pedipalpi. Sub-order a. Uropygi. Tribe 1. Urotricha. ,, 2. Tartarides. Sub-order b. Amblypygi. Order 3. Aranese. Sub-order a. Mesothelse. ,, b. Opisthothelaa. Tribe 1. Mygalomorphae. ,, 2. Arachnomorphse. Order 4. Palpigradi (= Microthelyphonida i. Orders. Solifugse (= Mycetophorse). Order 6. Pseudoscorpiones (= Chelonethi). Sub-order a. Panctenodactyli. ,, b. Hemictenodactyli. Order 7. Podogona (= Meridogastra). Order 8. Opiliones. Sub-order a. Laniatores. ,, b. Palpatores. ,, c. Anepignathi. Order 9. Rhynchostomi ( = Acari). Sub-order a. Cryptostigmata. ,, b. Metastigmata. ,, c. Prostigmata. ,, d. Astigmata. ,, e. Yermiformia., /. Tetrapoda. Class. ARACHNIDA.—Euartliropoda having two prosthomeres (somites which have passed from a post-oral to a prse-oral position), the appendages of the first represented by eyes, of the second by solitary rami which are rarely antenniform, more usually chelate. A tendency is exhibited to the fonnation of a metasomatic as well as a prosomatic carapace by fusion of the tergal surfaces of the somites. Intermediate somites forming a mesosoma occur, but tend to fuse superficially with the metasomatic carapace or to become co-ordinated with the somites of the metasoma, whether fused or distinct to form one region, the opisthosoma (abdomen of authors). In the most highly-developed forms the two anterior divisions (tagmata) of the body, prosoma and mesosoma, each exhibit six pairs of limbs, pediform and plate - like respectively, wdrilst the metasoma consists of six limbless somites and a post-anal spine. The genital apertures are placed in the first somite following the prosoma, excepting where a praegenital somite, usually suppressed, is retained. Little is known of the form of the appendages in the lowest archaic Arachnida, but the tendency of those of the prosomatic somites has been (as in the Crustacea) to pass from a generalized bi-ramose or multi-ramose form to that of uni-ramose antennse, chelae, and walking-legs. The Arachnida are divisible into two grades of structure—according to the fixity or non-fixity of the number of somites building up the body:— Grade A {of the Arachnida). ANOMOMEIIISTICA. —Extinct archaic Arachnida, in which (as in the Entomostracous Crustacea) the number of well-developed somites may be more or less than eighteen and may be grouped only as head (prosoma) and trunk or may be further differentiated. A telsonic tergal shield of greater or less size is always present, which may be imperfectly divided into well-marked but immovable tergites indicating incompletely differentiated somites. The single pair of palpiform appendages in front of the mouth has been found in one instance to be antenniform, whilst the numerous post-oral appendages in the same genus were bi-ramose. The position of the genital apertures is not known. Compound lateral eyes present; median eyes wanting. The body and head have the two pleural regions of each somite flattened and expanded on either side of the true gut-holding body-axis. Hence the name of the sub-class signifying tri-lobed, a condition realized also in the Xiphosurous Arachnids. The members of this group, whilst resembling the low*er Crustacea (as all lower groups of a branching genealogical tree must do), differ from them essentially in that the head exhibits only one prosthomere (in addition to the eye-bearing prosthomere) with palpiform appendages (as in all Arachnida) instead of two. The Anomomeristic Arachnida form a single sub-class, of which only imperfect fossil remains are known. Sub-class (of the Anomomeristica). TRILOBITZE.—The single sub-class Trilobitae constitutes the grade Anomomeristica. It has been variously divided into orders by a number of writers. The greater or less evolution and specialization of the metasomatic carapace appears to be the most important basis for classification— but this has not been made use of in the latest attempts at drawing up a system of the Trilobites. The form of the middle and lateral regions of the prosomatic shield has been used, and an excessive importance attached to the demarcation of certain areas in that structure. Sutures are stated to mark off some of these, pieces but in the proper sense of that term as applied to the skeletal structures