It must be confessed that we are singularly ignorant as to the functional significance of these remarkable organs—the entosternites. Their movement in an upward or downward direction in Limulus and Mygale must exert a pumping action on the blood contained in the dorsal arteries and the ventral veins respectively. In Scorpio the completion of the horizontal plate by oblique naps, so as to form an actual diaphragm shutting off the cavity of the prosoma from the rest of the body, possibly gives to the organs contained in the anterior chamber a physiological advantage in respect of the supply of arterial blood and its separation from the venous blood of the mesosoma. Possibly the movement of the diaphragm may determine the passage of air into or out of the lung-sacs. Muscular fibres connected with the suctorial pharynx are in Limulus inserted into the entosternite, and the activity of the two organs may be correlated.
5. The Blood and the Blood-vascular System.—The blood fluids of Limulus and Scorpio are very similar. Not only are the blood corpuscles of Limulus more like in form and granulation to those of Scorpio than to those of any Crustacean, but the fluid is in both animals strongly impregnated with the blue-coloured respiratory proteid, haemocyanin. This body occurs also in the blood of Crustacea and of Molluscs, but its abundance in both Limulus and Scorpio is very marked, and gives to the freshly-shed blood a strong indigo-blue tint.
 | Fig. 20.—View of the ventral surfaceof the mid-line of the prosomatic regionof Limulus polyphemus. The coxae ofthe five pairs of limbs following the cheliceraewere arranged in a series on eachside 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 ofchitinous plate, unpaired. mets, The right and left metasternites(corresponding to the similarly placedpentagonal sternite of Scorpio). Natural size. | |
| (After Lankester.) |
| Fig. 2l.—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.) |
 | Fig. 22.—Sectionthrough 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 Bournefrom Parker and Habwell’s Textbook of Zoology, Macmillan & Co.) |
| 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.
| 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 must cause a depression of the floor of the pericardium and a rising of the roof of the ventral blood sinus, and a consequent increase of volume and flow of blood to each. Whether the pericardium and the ventral sinus are made to expand simultaneously or all the movement is made by one only of the surfaces concerned, must depend on conditions of tension. In any case it is clear that we have in these muscles an apparatus for causing the blood to flow differentially in increased volume into either the pericardium, through the veins leading from the respiratory organs, or from the body generally into the great sinuses which bring the blood to the respiratory organs. These muscles act so as to pump the blood through the respiratory organs.