The following out of the consecutive clues, which one after the other arise in harmonious succession as the necessary sequence of the original working hypothesis, brings even now into view the manner in which the respiratory portion of the alimentary canal arose, and gives strong hints as to the position of that part of the arthropod which gave origin to the notochord. Here I will say no more at present, for the origin of the new alimentary canal of the vertebrate and of the notochord will be more fittingly discussed as a whole, after all the other organs of the vertebrate have been compared with the corresponding organs of the arthropod. [Illustration: FIG. 72.--DIAGRAM (UPPER HALF OF FIGURE) OF THE ORIGINAL POSITION OF VEINS (H) WHICH COME TOGETHER TO FORM THE HEART OF A VERTEBRATE. _C.N.S._, central nervous system; _nc._, notochord; _m._, myotome. The lower half of figure shows comparative position of the longitudinal venous sinus (_L.V.S._) in Limulus. _C.N.S._, central nervous system; _Al._, alimentary canal; _H._, heart; _m._, body-muscles.] The strong evidence that the vertebrate heart was formed from a pair of longitudinal venous sinuses on the ventral side of the central canal, carries with it the conclusion that the original single median dorsal heart of the arthropod is not represented in the vertebrate, {181}for the dorsal aorta cannot by any possibility represent that heart. Although it is not now functional the original existence of so important an organ as a dorsal heart may have left traces of its former presence; if so, such traces would be most likely to be visible in the lowest vertebrates, just as the median eyes are much more evident in them than in the higher forms. In Fig. 58 the position of the dorsal heart is shown in Limulus, and in Fig. 70 the shape and extent of this dorsal heart is shown. It extends slightly into the prosomatic region, and thins down to a point there, runs along the length of the animal and finally thins down to a point at the caudal end. The heart is surrounded by a pericardium, from which at regular intervals a number of dorso-ventral muscles pass, to be inserted into the longitudinal venous sinus on each side. These veno-pericardial muscles are absolutely segmental with the mesosomatic segments, and are confined to that region, with the exception of two pairs in the prosomatic region. Their homologies will be discussed later.