THE RELATIONSHIP OF THE CHELICERATE ARTERIAL SYSTEM ...

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30 THE JOURNAL OF ARACHNOLOGYleft dorsal suspensor musclesmesosomatic dorsoventral muscles,heartperineural vascular membronleg 5Fig. 22.-A: midsagittal view of Limulus polyphemus (class Merostomata), showing the endosternitesand the muscles which insert upon them, seen from the left. The muscles of the left side havebeen superimposed. The white circles in the mesosoma represent the locations of the mesosomaticendosternites. The two bumps on the dorsal surface of the cephalothoracic endosternite represent thepositions of the transverse suspensors. B: an enlarged detail, midsagittal, with musculature omitted;the region indicated by "see legend" contains Strands of connective tissue, in the adult, representingthe vestigial connection of the perineural vascular membrane to the endosternite. C: transverse sectionthrough the subesophageal ganglionic mass, showing the cephalothoracic endosternite and one pair ofdorsal suspensors, seen from the anterior.1885); these I Interpret äs homologous to the transverse suspensors of arachnid endosternites.Six abdominal endosternites are also present in Limulus (the mesosomatic entochondritesof Benham, 1885). Morphologically, each of these is a tendonified transversemuscle which is contractile at its lateral extremities (Fig. 23). Each one receives theinsertion of a singje pair of dorsal suspensors. The anatomical configuration of theseabdominal endosternites, in relation to the muscles which insert on them, gives theImpression that they are serially homologous to the metameric elements of the cephalothoracicendosternite, and that they represent a primitive stage of endosternite evolution,for this is the way that Lankester (1885) interpreted them. I do not doubt that themesosomatic dorsoventral muscles are serially homologous to those of the cephalothorax,but the transverse muscles differ from those of the cephalothorax in that: (1) they lieunder the nervous System instead of over it, and (2) they attach distally to movablestructures (the book gills) instead of the body wall. Lankester resolved the first problem,to his own satisfaction, by hypothesizing that the nerve cords (which he presumed wereprimitively in a lateral position) moved medially to their present position on the midline
FIRSTMAN-CHELICERATE ARTERIAL SYSTEM AND ENDOSTERNITE 31during a time when the endosternite was in a formative stage of evolutionary development.He assumed that the endosternites were derived from subepidermal connectivetissue of the ventral floor, and that the cephalothoracic endosternite (which presumablyis older) had arisen far enough from the ventral floor that it came to lie over the relocatednervous System, while the younger abdominal endosternites were still under it.Although Snodgrass (1952) feels that Lankester's hypothesis is overly contrived, I donot feel that the question of the morphological significance of the mesosomatic endosternitesof Limulus has been resolved one way or another. However, I am inclined to theopinion that the mesosomatic transverse muscles of Limulus, because they attach distallyto movable appendages, are not serially homologous to those which have been instrudorsalsuspensorstransversesuspensorsmesosomaticdorsoventralmusclesmesosomaticendosternitetelsoigillappendagenerve trun!
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