Page:EB1911 - Volume 02.djvu/317

This page has been proofread, but needs to be validated.
302
ARACHNIDA
An internal skeletal plate, the so-called “entosternite” of fibro-cartilaginous tissue, to which many muscles are attached, is placed between the nerve-cords and the alimentary tract in the prosoma of the larger forms (Limulus, Scorpio, Mygale). In the same and other leading forms a pair of much-coiled glandular tubes, the coxal glands (coelomocoels in origin), is found with a duct opening on the coxa of the fifth pair of appendages of the prosoma. The vascular system is highly developed (in the non-degenerate forms); large arterial branches closely accompany or envelop the chief nerves; capillaries are well developed. The blood-corpuscles are large amoebiform cells, and the blood-plasma is coloured blue by haemocyanin.
The alimentary canal is uncoiled and cylindrical, and gives rise laterally to large gastric glands, which are more than a single pair in number (two to six pairs), and may assume the form of simple caeca. The mouth is minute and the pharynx is always suctorial, never gizzard-like. The gonadial tubes (gonocoels or gonadial coelom) are originally reticular and paired, though they may be reduced to a simpler condition. They open on the first somite of the mesosoma. In the numerous degenerate forms simplification occurs by obliteration of the demarcations of somites and the fusion of body-regions, together with a gradual suppression of the lamelliferous respiratory organs and the substitution for them of tracheae, which, in their turn, in the smaller and most reduced members of the group, may also disappear.
The Eu-arachnida are divided into two grades with reference to the condition of the respiratory organs as adapted to aquatic or terrestrial life.

Grade a (of the Eu-arachnida). delobranchia (Hydropheustea).

Mesosomatic segments furnished with large plate-like appendages, the 1st pair acting as the genital operculum, the remaining pairs being provided with branchial lamellae fitted for breathing oxygen dissolved in water. The prae-genital somite partially or wholly obliterated in the adult. The mouth lying far back, so that the basal segments of all the prosomatic appendages, excepting those of the 1st pair, are capable of acting as masticatory organs. Lateral eyes consisting of a densely packed group of eye-units (“compound” eyes).
Order 1. Xiphosura.—The prae-genital somite fuses in the embryo with the prosoma and disappears (see fig. 19). Not free-swimming, none of the prosomatic appendages modified to act as paddles; segments of the mesosoma and metasoma ( = opisthosoma) not more than ten in number, distinct or coalesced.
Family—Limulidae (Limulus).
 ”   *Belinuridae (Belinurus, Aglaspis, Prestwichia).
 ”   *Hemiaspidae (Hemiaspis, Bunodes).


1911 Britannica-Arachnida-Limulus polyphemus13.png
Fig. 44.—Dorsal view of Limulus polyphemus, Latr.










(From Parker and Haswell, Text book of Zoology, after Leuckart.)








Remarks.—The Xiphosura are marine in habit, frequenting the shore. They are represented at the present day by the single genus Limulus (figs. 44 and 45; also figs. 7, 9, 11, to 15 and 20), often termed the king-crab, which occurs on the American coast of the Atlantic Ocean, but not on its eastern coasts, and on the Asiatic coast of the Pacific. The Atlantic species (L. polyphemus) is common on the coasts of the United States, and is known as the king-crab or horse-shoe crab. A single specimen was found in the harbour of Copenhagen in the 18th century, having presumably been carried over by a ship to which it clung.
A species of Limulus is found in the Buntersandstein of the Vosges; L. Walchi is abundant in the Oolitic lithographic slates of Bavaria.


1911 Britannica-Arachnida-Limulus polyphemus14.png

Fig. 45.—Ventral view of Limulus polyphemus.


1 to 6, The six prosomatic pairs of appendages.

abd, the solid opisthosomatic carapace.
tels, the post-anal spine (not the telson as the lettering would seem to imply, but only its post-anal
portion).

operc, the fused first pair of mesosomatic appendages forming the genital operculum.








(From Parker and Haswell, Text book of Zoology, after Leuckart.)










The genera Belinurus, Aglaspis, Prestwichia, Hemiaspis and Bunodes consist of small forms which occur in Palaeozoic rocks. In none of them are the appendages known, but in the form of the two carapaces and the presence of free somites they are distinctly intermediate between Limulus and the Trilobitae. The young form of Limulus itself (fig. 40) is also similar to a Trilobite so far as its segmentation and trilobation are concerned. The lateral eyes of Limulus appear to be identical in structure and position with those of certain Trilobitae.


1911 Britannica-Arachnida-Eurypterus Fischeri.png
Fig. 46.—Eurypterus Fischeri, Eichwald. Silurian of Rootzikil. Restoration after Schmidt.
The dorsal aspect is presented showing the prosomatic shield with paired compound eyes
and the prosomatic appendages II. to VI. The small first pair of appendages is concealed
from view by the carapace, 1 to 12 are the somites of the opisthosoma; 13, the post-anal
spine.









(From Zittel’s Text-book of Palaeontology, The Macmillan Co., New York, 1896.)










Order 2. Gigantostraca (figs. 46, 47).—Free-swimming forms, with the appendages of the 6th or 5th and 6th pairs flattened or lengthened to act as oars; segments of mesosoma and metasoma ( = opisthosoma), twelve in number.