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1911 Britannica-Arachnida-Limulus polyphemus10.png

Fig. 20.—View of the ventral surface of the mid-line of the prosomatic region of Limulus polyphemus. The coxae of the
five pairs of limbs following the chelicerae were arranged in a series on each side between the mouth, M, and the
metasternites, mets.

sf, The sub-frontal median sclerite.
Ch, The chelicerae.
cam, The camerostome or upper lip.
M, The mouth.
pmst, The promesosternal sclerite of chitinous plate, unpaired.

mets, The right and left metasternites (corresponding to the similarly placed pentagonal sternite of Scorpio). Natural size.

(After Lankester.)

1911 Britannica-Arachnida-Laurie.png
Fig. 21.—Development of the lateral eyes of a scorpion. h, Epidermic cell-layer; mes, mesoblastic connective tissue; n, nerves; II, III, IV, V, depressions of the epidermis
in each of which a cuticular lens will be formed.

(From Korschelt and Heider, after Laurie.)

1911 Britannica-Arachnida-Bourne.png

Fig. 22.—Section through the lateral eye of Euscorpius italicus.

lens, Cuticular lens.
nerv. c, Retinal cells (nerve-end cells).
rhabd, Rhabdomes.
nerv. f, Nerve fibres of the optic nerve.

int, Intermediate cells (lying between the bases of the retinal cells).

(After Lankester and Bourne from Parker and Habwell’s Text book of Zoology, Macmillan & Co.)

1911 Britannica-Arachnida-Watase.png
Fig. 23.—Section through a portion of the lateral eye of Limulus, showing three ommatidia—A, B and C. hyp, The epidermic cell-layer (so-called hypodermis), the cells
of which increase in volume below each lens, l, and become nerve-end cells or retinula-cells, rt; in A, the letters rh point to a rhabdomere secreted by the cell rt; c, the
peculiar central spherical cell; n, nerve fibres; mes, mesoblastic skeletal tissue; ch, chitinous cuticle.

(From Korschelt and Heider after Watase.)

The great dorsal contractile vessel or “heart” of Limulus is closely similar to that of Scorpio; its ostia or incurrent orifices are placed in the same somites as those of Scorpio, but there is one additional posterior pair. The origin of the paired arteries from the heart differs in Limulus from the arrangement obtaining in Scorpio, in that a pair of lateral commissural arteries exist in Limulus (as described by Alphonse Milne-Edwards (6)) leading to a suppression of the more primitive direct connexion of the four pairs of posterior lateral arteries and of the great median posterior arteries with the heart itself (fig. 29). The arterial system is very completely developed in both Limulus and Scorpio, branching repeatedly until minute arterioles are formed, not to be distinguished from true capillaries; these open into irregular swollen vessels which are the veins or venous sinuses. A very remarkable feature in Limulus, first described by Owen, is the close accompaniment of the prosomatic nerve centres and nerves by arteries, so close indeed that the great ganglion mass and its out-running nerves are actually sunk in or invested by arteries. The connexion is not so intimate in Scorpio, but is nevertheless a very close one, closer than we find in any other Arthropods in which the arterial system is well developed, e.g. the Myriapoda and some of the arthrostracous Crustacea. It seems that there is a primitive tendency in the Arthropoda for the arteries to accompany the nerve cords, and a “supra-spinal” artery—that is to say, an artery in close relation to the ventral nerve cords—has been described in several cases. On the other hand, in many Arthropods, especially those which possess tracheae, the arteries do not have a long course, but soon open into wide blood sinuses. Scorpio certainly comes nearer to Limulus in the high development of its arterial system, and the intimate relation of the anterior aorta and its branches to the nerve centres and great nerves, than does any other Arthropod.
An arrangement of great functional importance in regard to the venous system must now be described, which was shown in 1883 by Lankester to be common to Limulus and Scorpio. This arrangement has not hitherto been detected in any other class than the Arachnida, and if it should ultimately prove to be peculiar to that group, would have considerable weight as a proof of the close genetic affinity of Limulus and Scorpio.

1911 Britannica-Arachnida-Heider.png

Fig. 24.—Diagrams of the development and adult structure of one of the paired central eyes of a scorpion.

A, Early condition before the lens is deposited, showing the folding of the epidermic cell-layer into three.
B, Diagram showing the nature of this infolding.
C, Section through the fully formed eye.
h, Epidermic cell-layer.
r, The retinal portion of the same which, owing to the infolding, lies between gl, the corneagen or lens-forming portion, and pr, the post-retinal or capsular portion or fold.
l, Cuticular lens.
g, Line separating lens from the lens-forming or corneagen cells of the epidermis.
n, Nerve fibres.
rh, Rhabdomeres.

[How the inversion of the nerve-end-cells and their connexion with the nerve-fibres is to be reconciled with the condition found in the adult, or with that of the
monostichous eye, has not hitherto been explained.]

(From Korschelt and Heider.)

The great pericardial sinus is strongly developed in both animals. Its walls are fibrous and complete, and it holds a considerable volume of blood when the heart itself is contracted. Opening in pairs in each somite, right and left into the pericardial sinus are large veins, which bring the blood respectively from the gill-books and the lung-books to that chamber, whence it passes by the ostia into the heart. The blood is brought to the respiratory organs in both cases by a great venous collecting sinus having a ventral median position. In both animals the wall of the pericardial sinus is connected by vertical muscular bands to the wall of the ventral venous sinus (its lateral expansions around the lung-books in Scorpio) in each somite through which the pericardium passes. There are seven pairs of these veno-pericardiac vertical muscles in Scorpio, and eight in Limulus (see figs. 30, 31, 32). It is obvious that the contraction of these muscles