Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/578

 528

ARACHNIDA

(3) of these entosternites for the purpose of comparison, and also ascertained the relations of the very numerous muscles which are inserted into them (4). The entosternites are cartilaginous in texture, but they have neither the chemical character nor the microscopic structure of the hyaline cartilage of Vertebrates. They yield chitin in place of chondrin or gelatin—as does also the cartilage of the Cephalopod’s endoskeleton. In microscopic structure they all present the closest agreement with one another. We find a firm, homogeneous or sparsely fibrillated matrix in which are embedded nucleated cells (corpuscles of protoplasm) arranged in rows of three, six, or eight, parallel with the adjacent lines of fibrillation. A minute entosternite having the above-described structure is found in the Crustacean Apus between the bases of the mandibles, and also in the Decapoda in a similar position, but in no Crustacean does it attain to any size or importance. On the other hand, the entosternite of the Araclmida is a very large and important feature in the structure of the prosoma, and must play an important part in the economy of these organisms. In Limulus (Figs. 1 and 2) it has as many as twenty-five pairs of muscles attached to it, coming 6 /I ^1 -v ^ from the bases of the surrounding limbs and from the dorsal carapace and from the pharynx. It consists of an oblong plate two inches in length and one in breadth, with a pair of tendinous outgrowths standing out from it at right angles on each side. It x- 28.—The . “floats” between the 00 ™ right2coxal5gland of Limulus polyFio. phemus, Latr. a to a, Posterior borders of prOSOmatlC nerve the chitinous bases of the coxre of the second, „ ai. T, n ^ +t,q third, fourth, and fifth prosomatic limbs; 6, centres ana tne longitudinal lobe or stolon of the coxal gland ; alimentary Canal. In c, its four transverse lobes or outgrowths , ., . corresponding to the four coxse. (From Lan- 6acn SOmite Ot tne tester, loc. cit., after Packard.) meSOSOma is a small, free entosternite having a similar position, but below or ventrad of the nerve cords, and having a smaller number of muscles attached to it. The entosternite was probably in origin part of the fibrous connective tissue lying close to the integument of the sternal surface—giving attachment to muscles corresponding more or less to those at present attached to it. It became isolated and detached, why or with what advantage to the organism it is difficult to say, and at that period of Arachnidan development the great ventral nerve cords occupied a more lateral position than they do at present. We know that such a lateral position of the nerve cords preceded the median position in both Arthropoda and Chsetopoda. Subsequently to the floating off of the entosternite the approximation of the nerve cords took place in the prosoma, and thus they were able to take up a position below the entosternite. In the mesosoma the approximation had occurred before the entosternites were formed. In the Scorpion (Figs. 3 and 4) the entosternite has tough membrane-like outgrowths which connect it with the body-wall, both dorsally and ventrally forming an oblique diaphragm, cutting off the cavity of the prosoma from that of the mesosoma. It was described by Newport as “the diaphragm.” Only the central and horizontal parts of this structure correspond precisely to the entosternite of Limulus : the right and left anterior processes (marked ap in Figs. 3 and 4, and RAP, LAP, in Figs. 1 and 2) correspond in the two animals, and the median

lateral process Imp of the Scorpion represents the tendinous outgrowths ALR, PLR of Limulus. The Scorpion’s entosternite gives rise to outgrowths, besides the great posterior flaps, pf, which form the diaphragm, unrepresented in Limulus. These are a ventral arch forming a neural canal through which the great nerve cords pass (Figs. 3 and 4, snp), and further a dorsal gastric canal and arterial canal which transmit the alimentary tract and the dorsal artery respectively (Figs. 3 and 4, GC, DR). In Limulus small entosternites are found in each somite of the appendage-bearing mesosoma, and we find in Scorpio, in the only somite of the mesosoma which has a welldeveloped pair of appendages, that of the pectens, a small entosternite with ten pairs of muscles inserted into it. The supra-pectinal entosternite lies ventrad of the nerve cords. In Mygale (Figs. 5 and 6) the form of the entosternite is more like that of Limulus than is that of Scorpio. The anterior notch Ph.N. is similar to that in Limulus, and the three pairs of upstanding tendons on the dorsal surface correspond to the two similar pairs in Limulus, whilst the imbricate triangular pieces of the posterior median region resemble the similarly-placed structures of Limulus in a striking manner. 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 flaps, 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 vi of the supply of arterial blood and its separation from the "Wi 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 of the arterial Blood-vascular System.—The Fisystem y 29. —ofDiagram A, Scorpio, and B, blood fluids of Limulus and Limulus. The Roman numerals the body somites and the Scorpio are very similar. indicate two figures are adjusted for comNot only are the blood cor- parison. ce, Cerebral arteries; sp, or medullary artery; puscles of Limulus more like supra-spinal c, caudal artery; 1, lateral anastoartery of Limulus. The figure in form and granulation to motic B also shows the peculiar neural those of Scorpio than to those investiture formed by the cerebral arteries Limulus and the derivaof any Crustacean, but the tion frominthis of the arteries to the fluid is in both animals limbs, III, IV, VI, whereas in Scorpio the latter a separate strongly impregnated with origin from the have anterior aorta. Lankester, “Limulus an the blue-coloured respiratory (From Arachnid.”) proteid Hsemocyanin. 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. 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