ENDOSKELETAL SYSTEMS OF LIMULUS AND SCORPIO. 363 
The ventral longitudinal muscles of the mesosoma are retained, whilst the metasoma 
is altogether in a reduced or degenerate condition. 
One special feature resulting from the concentration of the skeletal covering of 
Limulus in two great tergal plates (the anterior and posterior carapaces) is the deve- 
lopment of muscles connected with the limbs and other sternal parts of the mesosoma, 
which do not keep within the limits of the segment to which one end of such muscle 
may be attached, but take a long course forwards so as to receive their dorsal attach- 
ment either in the prosomatic carapace or in portions of the meso-metasomatic carapace 
anterior to (in other cases posterior to) the segment of their insertion. Such vertical- 
oblique muscles may possibly in some cases be regarded as resulting from dislocations 
of normal vertical muscles, which normally arise, and are inserted in the tergum and 
sternum of one and the same segment. But, generally speaking, it seems necessary to 
regard such muscles as NEw developments, since it is difficult to imagine the steps by 
which a muscle (apart from a movement of the hard pieces of tergum or sternum to 
which it was primitively attached) could acquire new attachments outside the segment 
to which it properly belongs. 
The great entapophyses (Hnt') of the prosoma of Limulus appear to furnish an 
important instance of the removal of a skeletal piece from association with the segment 
to which it primitively belonged, and of its incorporation by fusion with a segment tn* 
front of it. Such a transference is familiar enough in the higher vertebrata in the case 
of the transference of the centrum of the atlas vertebra to the next following segment, 
where it appears as the odontoid process of the axis-vertebra. Apparently in the same 
way the entapophyses of the first mesosomatic segment of Limulus have been detached 
from the rest of the dorsal sclerite of the first mesosomatic segment and have been incor- 
porated with the prosomatic carapace, thus taking up a position in front of the great 
tergal hinge to which they were primitively posterior. The attachment of muscles to 
these large entapophyses, and the serial relations of those muscles and of the muscles 
attached to the area of the first segment of the mesosoma and of the succeeding meso- 
somatic segments, is decidedly in favour of the supposition that such a transference 
has taken place. The attachment of the external branchials of Limulus (20), and of 
the two most anterior of the mesosomatic dorso-ventrals (12), gives important evidence 
in this matter. When we adopt the view that the prosomatic entapophyses are really 
the entapophyses of the first or genital segment of the mesosoma, and that correspond- 
ingly the following five pairs of entapophyses belong respectively to the five succeeding 
segments of the mesosoma, whilst the sixth pair of entapophyses belongs to the meta~ 
soma, the interpretation of the series of muscles attached to them and to the adjacent 
parts of the tergum becomes intelligible. This view corresponds with that which I 
had put forward as the result of a study of the hard parts only, in my article “Limulus 
an Arachnid” (Q. J. Micr. Sci. 1881). See woodcut, fig. 1, of the tergal surface of 
Limulus in that article. 
