370 PEOF. E. E. LANKESTEB ON THE MUSCULAE AND 



secondary (or newly originated) muscles, would furnish ample field for speculation and 

 ingenuity. An important difference between the musculature of the coxo-stemal joint 

 of the prosomatic limbs of Limuhis 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 protomei'ite (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 54, and from 

 the two following entapophyses 56, 72 (PI. LXXV.), 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 coxae 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 

 (PI. 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 (PI. LXXVI1I. 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 

 (PI. 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 JAmulus 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 

 Scorpio, 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 



