PROF. E. R. LANKESTER ON LIMULUS AND SCORPIO. 561 
Part IV. Comparison of the Muscular and Endoskeletal Systems of Limulus and 
Scorpio, and Consideration of the Morphological Significance of the Facts recorded. 
By E. Ray LanKester. 
I HAVE not attempted to modify the descriptions of the hard parts and the muscles of 
Limulus given by Mr. Benham, and of Scorpio given by Miss Beck, so as to obtain a 
uniformity of nomenclature and numbering for these parts in the two animals. Unde- 
niably the nomenclature which has seemed to be the simplest and most intelligible is 
at the same time exceedingly clumsy, but that is a reproach which is equally justified 
in the case of all attempts at the naming of muscles. We have before us, in Parts II. 
and III. of this Memoir, a systematic description of the muscles of the King Crab and 
the Scorpion which can serve as a basis for comparisons between the two animals. 
Like all Arthropoda these animals have lost entirely the circular muscular layer of 
the body-wall which their Vermian Chetopod-like ancestors possessed’. This suppres- 
sion of the circular muscular layer is correlated with the development of the great 
tergal and sternal sclerites, and the consequent incompressibility of the body. The 
muscles of the body-wall are entirely longitudinal or else dorso-ventral. The muscles 
are practically divisible into three great groups, viz. those of the body-wall just referred 
to, those of the limbs, and those of the viscera (pharyngeal and veno-pericardiacs, 
and intrinsic muscles). Those which pass from one or other part of the body-wall to 
the limbs are by far the most bulky and numerous group of the three, in both Limulus 
and Scorpio. 
Neither Limulus nor Scorpio is in a primitive or archaic condition, so far as the 
segmentation of the body and the differentiation of its appendages are concerned. We. 
have not to deal in either case with a simple condition, but in each with a highly 
specialized condition. Presumably the ancestors of the Arachnida, in which class both 
Limulus and Scorpio find their places, were provided with a completely segmented 
body, consisting of at least eighteen separately movable somites, a prostomium, and a 
postanal spine. Each of the eighteen segments at a remote period of the ancestry 
carried a pair of appendages. The musculature at this time was no doubt very simple 
and regular, exactly repeating itself in each successive body-ring. Assuming that 
dorsal and ventral sclerites had been developed on each segment, and that the coxe of 
the limbs were chitinized, it is yet probable that at such an early stage no represen- 
tatives of the remarkable floating cartilages or “ entochondrites,” which form so charac- 
teristic a feature in the organization of living Arachnida, had yet been brought into 
existence. The simple musculature may be supposed to have consisted of—I, a series 
of paired dorsal longitudinal muscles passing from tergite to tergite of each successive 
’ Unless the latero-dorsal muscles of the mesosoma of Scorpions (15 to 20 in Miss Beck’s enumeration) may 
be considered as representing that layer. 
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