Page:EB1911 - Volume 02.djvu/314

 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 Laemodipoda 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 arabic 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.





Grade A. ANOMOMERISTICA.
 * Sub-Class. TRILOBITAE.
 * Orders. Not satisfactorily determined.

Grade B. NOMOMERISTICA.
 * Sub-Class I. PANTOPODA.
 * Order 1. Nymphonomorpha.
 * Order 2. Ascorhynchomorpha.
 * Order 3. Pycnogonomorpha.
 * Sub-Class II. EU-ARACHNIDA.
 * Grade a., Lankester (vel , Pocock).
 * Order 1. Xiphosura.
 * Order 2. Gigantostraca.
 * Grade b., Lankester (vel , Pocock).
 * Section . Pectinifera.
 * Order 1. Scorpionidea.
 * Sub-order a. Apoxypoda.
 * Sub-order b. Dionychopoda.
 * Section . Epectinata.}
 * Order 2. Pedipalpi.
 * Sub-order a. Uropygi.
 * Tribe 1. Urotricha.
 * Tribe 2. Tartarides.
 * Sub-order b. Amblypygi.
 * Order 3. Araneae.
 * Sub-order a. Mesothelae.
 * Sub-order b. Opisthothelae.
 * Tribe 1. Mygalomorphae.
 * Tribe 2. Arachnomorphae.
 * Order 4. Palpigradi (= Microthelyphonidae).
 * Order 5. Solifugae (= Mycetophorae).
 * Order 6. Pseudoscorpiones (= Chelonethi).
 * Sub-order a. Panctenodactyli.
 * Sub-order b. Hemirtenodactyli.
 * Order 7. Podogona (= Ricinulel).
 * Order 8. Opiliones.
 * Sub-order a. Laniatores.
 * Sub-order b. Palpatores.
 * Sub-order c. Anepignathi.
 * Order 9. Rhynchostomi (= Acari).
 * Sub-order a. Notostigmata.
 * Sub-order b. Cryptostigmata.
 * Sub-order c. Metastigmata.
 * Sub-order d. Prostigmata.
 * Sub-order e. Astigmata.
 * Sub-order f. Vermiformia.
 * Sub-order g. Tetrapoda.

.—Euarthropoda having two prosthomeres (somites which have passed from a post-oral to a prae-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 formation 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, whilst 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 antennae, 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). ANOMOMERISTICA.—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 lower 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). TRILOBITAE'''.—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 of the Vertebrata, no sutures exist in the chitinous cuticle of Arthropoda. That any partial fusion of originally distinct chitinous plates takes place in the cephalic shield of Trilobites, comparable to the partial fusion of bony pieces by suture in Vertebrata, is a suggestion contrary to fact.

The Trilobites are known only as fossils, mostly Silurian and prae-Silurian; a few are found in Carboniferous and Permian strata. As many as two thousand species are known. Genera with small metasomatic carapace, consisting of three to six fused segments distinctly marked though not separated by soft membrane, are Harpes, Paradoxides and Triarthrus (fig. 34). In Calymene, Homalonotus and Phacops (fig. 38) from six to sixteen segments are clearly marked by ridges and grooves in the metasomatic tagma, whilst in Illaenus the shield so formed is large but no somites are marked out on its surface. In this genus ten free somites (mesosoma) occur between the prosomatic and metasomatic carapaces. Asaphus and Megalaspis (fig. 39) are similarly constituted. In Agnostus (fig. 40) the anterior and posterior carapaces constitute almost the entire body, the two carapaces being connected by a mid-region of only two free somites. It has been held that the forms with a small number of somites marked in the posterior carapace and numerous free somites between the anterior and posterior carapace, must be considered as anterior to those in which a great number of posterior somites are traceable in the metasomatic carapace, and that those in which the traces of distinct somites in the posterior or metasomatic carapace are most completely absent must be regarded as derived from those in which somites are well marked in the posterior