{137}This plastron, it is true, is found in other animals, all of which are members of the scorpion tribe, except in one instance, and this, strikingly enough, is the crustacean Apus--a strange primitive form, which is acknowledged to be the nearest representative of the Trilobita still living on the earth. None of these forms, however, possess any sign of an internal cartilaginous branchial skeleton, such as is possessed by Limulus. Scorpions, Apus, Limulus, are all surviving types of the stage of organization which had been reached in the animal world when the vertebrate first appeared. THE MESOSOMATIC OR RESPIRATORY SKELETON OF LIMULUS, COMPOSED OF SOFT CARTILAGE. Searching through the literature of the histology of the cartilaginous tissues in invertebrate animals, to see whether any cartilage had been described similar to that seen in the branchial cartilages of Ammocoetes, and whether such cartilage, if found, arose in a fibrous tissue resembling muco-cartilage, I was speedily rewarded by finding, in Ray Lankester's article on the tropho-skeletal tissues of Limulus, a picture of the cartilage of Limulus, which would have passed muster for a drawing of the branchial cartilage of Ammocoetes. This clue I followed out in the manner described in my former paper in the _Journal of Anatomy and Physiology_, and mapped out the topography of this remarkable tissue. Limulus, like other water-dwelling arthropods, breathes by means of gills attached to its appendages. These gill-bearing appendages are confined to the mesosomatic region, as is seen in Fig. 59; and these appendages are very different to the ordinary locomotor appendages, which are confined to the prosomatic region. Each appendage, as is seen in Fig. 58, consists mainly of a broad, basal part, which carries the gill-book on its under surface; the distal parts of the appendage have dwindled to mere rudiments and still exist, not for locomotor purposes, but because they carry on each segment organs of special importance to the animal (see Chapter XI.). As is seen in Fig. 58, the basal parts of each pair of appendages form a broad, flattened paddle, by means of which the animal is able to swim in a clumsy fashion. Very striking and suggestive is the difference between these gill-bearing mesosomatic appendages and the non-gill-bearing locomotor appendages of the prosoma. {138}[Illustration: FIG. 58.--TRANSVERSE SECTION THROUGH THE MESOSOMA OF LIMULUS, TO SHOW THE ANTERIOR (A) AND THE POSTERIOR (B) SURFACES OF A MESOSOMATIC OR BRANCHIAL APPENDAGE. In each figure the branchial cartilaginous bar, _Br.C._, has been exposed by dissection on one side. _Ent._, entapophysis; _Ent.l._, entapophysial ligament cut across; _Br.C._, branchial cartilaginous bar, which springs from the entapophysis; _H._, heart; _P._, pericardium; _Al._, alimentary canal; _N._, nerve cord; _L.V.S._, longitudinal venous sinus; _Dv._, dorso-ventral somatic muscle; _Vp._, veno-pericardial muscle.]