370 PROF. E. R. LANKESTER ON THE MUSCULAR AND 
secondary (or newly originated) muscles, would furnish ample field for speculation and 
ingenuity. An important difference between the musculature of the coxo-sternal joint 
of the prosomatic limbs of Limulus and Scorpio is to be found in the fact that in 
Scorpio, as shown by Miss Beck, we have muscles arising from the prosomatic tergite, 
and inserted into the deutomerites of some of the limbs (106, 107, 108), and others 
arising from the ventral entosclerites also inserted into the deutomerites of limbs (100, 
101, 102, 103), whereas in Limulus no deutomerite appears to receive any muscle from 
the body-wall, such muscles being confined to the protomerite (coxa). 
Muscles arising from the Plastron or Prosomatic Entochondrite in Limulus and 
Scorpio.— When we exclude the obliquely-running antero-posterior muscles, which pass 
from the tergum of the mesosomatic carapace of Limulus to the prosomatic plastron, 
namely, the great dorso-entapophysial (1) and its branches 83, 84, 85, 86, 87, those 
from the great entapophyses (originally part of the mesosoma) 53 and 64, and from 
the two following entapophyses 56,72 (Pl. LX XV.), we find that the muscles attached 
to this body in both Limulus and Scorpion are practically the typical muscles of the 
ventral surface of the prosoma, and are strictly comparable in the two animals. At 
either end we find longitudinal ventral muscles (anteriorly in Scorpio only) connected 
with similar entochondrites in the following segments: dorsally are representatives of the 
dorso-ventral segmental muscles proper to the prosoma; and laterally muscles attached 
to the cox of the prosomatic limbs. There are three pairs of vertical tergoplastral 
muscles in the Scorpion (63, 64, 65), and these correspond to the muscle 52 of Limulus 
(Pl. LXXV.), and have no other representative, their place being taken by oblique 
muscles, whose tergal attachment is in the mesosoma. Possibly, however, the muscle 
64 of the Scorpion (Pl. LXXVIII. figs. 6 & 7) may be considered as the equivalent of 
the muscles 53 and 54 of Limulus,in which case we should have to suppose that in the 
Scorpion, as well as in Limulus, the fusion of a piece of the first mesosomatic tergite 
with the prosomatic carapace had taken place. There seems to be no representative 
in the Scorpion of the anterior vertical tergoplastral muscles of Limulus, 49, 50, 51 
(Pl. LXXV.), and in fact the most anterior region of the plastron in the Scorpion, as 
has been before pointed out, appears to stop short of the anterior region of the 
prosoma, whereas in Limulus it reaches anteriorly to the full limit of the ventral 
surface. 
When we compare the muscles passing from the plastron to the limbs in Limulus 
and Scorpio, we find again reason to suppose that the plastron of the latter does not 
comprise the ventral surface of the first prosomatic segment, although it does do so in 
Limulus. In fact, we have no muscles from the plastron to the first pair of limbs in 
Scorpro, whilst in Limulus we have two such muscles, viz. Nos. 30 and 31. In place of 
these, in Scorpio we have muscles which arise from the preoral entosclerite (96, 97, 98). 
Contrariwise there are no muscles of ventral origin in Limulus which are inserted into 
