(971). Professor Muller* has accurately described the pulmonibranchiae as formed of a multitude of closely-approximated, thin, double lamellae, which communicate, by a small orifice in each, with the external air admitted into a common cavity through the spiracle on the surface of the body. The blood, distributed through these lamellae, is brought into contact with the air in their interior through their membranous structure. The minute anatomy of these lamellae, and the manner in which they are permeated by the blood, afford some points of interest. Each side of these double lamellae is formed of an exceedingly delicate and apparently structureless double membrane, which includes within it a parenchymatous tissue formed of single vesicles or cells. The convex margin of each lamella is bounded by a delicate but distinct vessel, which seems to form the means of intercommunication with the anastomosing network of vessels distributed over the branchiae, since the delicate evanescent vessels traced into the lamellae are derived from those which bound their convex margin.

(972). At the posterior part of the inner side of the branchiae, where the lamellae are covered by the thick membrane and peritoneum that lines the common cavity of the branchiae, there are several small orifices, the commencement of vessels which afterwards, when collected together, form the larger channels that convey back the blood to the heart. These vessels form delicate trunks or sinuses, which pass around the sides of the body in the posterior part of each segment, and, becoming gradually enlarged by communicating with other vessels in their progress, pour their contents into the heart at the auricular orifices upon its dorsal aspect.

* Meckel's Archiv, 1828.

(973). The following, then, will be the course of the circulation in the Macrourous Arachnidans. The blood received by the veins from the branchiae is conveyed to the heart round the sides of the segments, receiving accessions from other vessels in the segments in its course, and enters the heart at the posterior part of each chamber through the orifices of Straus-Durckheim. The auriculo-ventricular cavity, dilated by the influx of blood, begins first to contract by the action of the circular fibres at the posterior part of each chamber. By this contraction, part of the blood is at once propelled laterally, through the systemic arteries, to the interior and sides of the body, while the remaining and chief portion is forced onwards, through the valves and body of the chamber, by the successive contraction of the circular fibres, into the next chamber. A fresh accession of blood enters the heart at the auricular orifices in the short interval of time that elapses between the contractile actions of the two chambers, which interval is probably occasioned by the reaction of the lateral muscular appendages of the organ.

These contractions are carried gradually onwards through the whole of the succeeding segments; so that before a third chamber has contracted, the first is again filled and ready to be emptied, thus occasioning, by their alternate movements, those pulsatory motions which are so well known in the dorsal vessel of Insects. The blood, propelled by these successive contractions through the aorta, is distributed to the organs in the head and thorax and the organs of locomotion. Part of it is also sent round the aortic arches through the supra-spinal artery backwards into the abdomen, giving off its minute currents for the nourishment of the nervous ganglionic cord, while another portion, intermingled with that collected in the portal vessels, is sent to the branchiae. But its principal current still flows in the supra-spinal artery, along the upper surface of the cord to the terminal ganglion of the tail, where it divides into four streams, two of which go out at the sides of the ganglion to nourish the segment, while the other two, now greatly reduced in size, proceed backwards along the terminal nerves of the cord, and, becoming more and more subdivided in the last segment of the tail, are diffused through the surrounding structures.

These form minute anastomoses with numerous small vessels, which, gradually collecting in separate trunks on the under surface of the last segment, form the origin of the caudal portion of the sub-spinal vessel which conveys the returning blood forwards from the tail to the abdomen, to be aerated in the branchiae before it is again transmitted to the heart. In like manner, the blood that has already circulated through the organs of locomotion, the cephalothorax, and abdomen, appears to be collected in the veins which transmit it to the branchiae before it is again employed in the circulation. Throughout the whole of its course along the artery in the tail, the blood is passed in small currents into the sub-spinal vessel, - thus intermingling the venous and arterial blood, precisely as occurs in the abdomen. But the circulation in the caudal prolongation of the heart yet remains to be explained. We have already seen that the great dorsal artery in the tail, above the colon, forms direct vascular communications around its sides with the sub-spinal vessel upon the ventral surface, in which the course of the blood is propelled forwards to the abdomen.

It is certain, therefore, that the action of the great chamber of the heart must impel the blood at once in every direction, chiefly forwards, and laterally, but also in part backwards through the caudal artery; otherwise it would be impossible for this structure to form its anastomoses with the sub-spinal vein without occasioning two opposing currents in the same vessel.

(974). In the nervous system of Spiders we observe that progressive concentration of the nervous centres, which we have traced through the lower forms of the Homogangliata, carried to the utmost extent. Spiders are appointed destroyers of insects, with which they maintain cruel and unremitting warfare. That the destroyer should be more powerful than the victim is essential to its position; that it should excel its prey in cunning and sagacity is likewise a necessary consequence; and by following out the same principles which have already been so often insisted upon, concerning the inseparable connexion that exists between the perfection of an animal and the centralization of its nervous ganglia, we find in the class before us an additional confirmation of this law. In Scorpions, indeed, the nervous masses composing the ventral chain of ganglia are still widely separated, especially those situated in the segments of the tail: in the cephalo-thorax they are of proportionately larger dimensions, and moreover exhibit this remarkable peculiarity, that, instead of being-united by two cords of communication, there are three interganglionic nerves connecting each division.