[Illustration: FIG. 68.--DIAGRAM OF THREE BRANCHIAL SEGMENTS OF AMMOCOETES (A) COMPARED WITH THREE BRANCHIAL SEGMENTS AFTER TRANSFORMATION (B) TO SHOW HOW THE BRANCHIAL APPENDAGES OF AMMOCOETES FORM THE BRANCHIAL POUCHES OF PETROMYZON. (After NESTLER.) In both figures the branchial cartilages (_br. cart._), the branchial view (_V. br._), and the sense-organs (_S_), are marked out in order to show corresponding points. The muscles, blood-spaces, branchial arteries, etc., of each branchial segment are not distinguished, being represented a uniform black colour. _Bro._, the bronchus into which each gill-pouch opens.] In the case of the scorpions, various suggestions have been made as to the manner in which such a conversion may have taken place. The most probable explanation is that given by Macleod, in which {170}each of the branchiæ of the scorpion group is directly compared with the branchial part of the Limulus appendage which has sunk into and amalgamated with the ventral surface. According to this view, the modification which has taken place in transforming the branchial Limulus-appendage into the branchial scorpion-appendage is a further stage of the process by which the Limulus branchial appendage itself has been formed, viz. the getting rid of the free locomotor segments of the original appendage, thus confining the appendage more and more to the basal branchial portion. So far has this process been carried in the scorpion that all the free part of the appendage has disappeared; apparently, also, the intrinsic muscles of the appendage have vanished, with the possible exception of the post-stigmatic muscle, so that any direct comparison between the branchial appendages of Limulus and the scorpions is limited to the comparison of their branchiæ, their nerves, and their afferent and efferent blood-vessels. In the case of Ammocoetes the comparison must be made not with air-breathing but with water-breathing scorpions, such as existed in past ages in the forms of Eurypterus, Pterygotus, Slimonia, and with the crowd of trilobite and Limulus-like forms which were in past ages so predominant in the sea; forms in some of which the branchial appendages had already become internal, but which, from the very fact of these forms being water-breathers, probably resembled, in respect of their respiratory apparatus, Limulus rather than the present-day scorpion. On the assumption that the branchial appendages of Ammocoetes, like the branchial appendages of the scorpion group, are to a certain extent comparable with those of Limulus, it becomes a matter of great interest to inquire whether the mode in which respiration is effected in Ammocoetes resembles most that of Limulus or of the scorpion. THE ORIGIN OF THE BRANCHIAL MUSCULATURE.