STUDIES ON THE ARACHNID EN'l'OSTEKNITE. 247 



III. Theories of the Origin of the Entosternite. 



As long ago as 1881 Laukester (5), wlieu describing the 

 entosternite of Limulus, said it may bo regarded as an en- 

 largement and interlacing of the respective tendons of the 

 muscles which are attached to it. By implication a similar 

 origin was predicated of the entosternites of Scorpions and 

 Spiders. This opinion was accepted by Schimkewitsch (9, 

 10) in the case of the entosternite of Scorpions, Spiders, and 

 other air-breathing Arachnids, and for that of Liniulus by 

 Bernard (3), who, however, regarded it solely as a derivative 

 of the ventral longitudinal muscle-bands. Bernard's views as 

 to the origin of the entosternite in the terrestrial Arachnids, 

 which he considers to be in no way related to Limulus and 

 its extinct allies, will be referred to later on. 



It appears to me that the evidence in favour of Lankester's 

 view of the mode of production of this plate in both groups 

 of Arachnids is overwhelming; a comparative study of the 

 entosternites in this class precludes, to my mind, any other 

 hypothesis as to their source. 



What muscles, then, have taken the largest share in their 

 furination ? 



There is reason to believe that the prosoma was originally 

 supplied with five pairs of tergo-sternal (dorso-ventral) 

 muscles serially repeating those of the opisthosoma, and 

 passing vertically from the under surface of the carapace to 

 be inserted ventrally on the sternum close to the points of 

 articulation of the post-oral appendages. There Avere also a 

 dorsal and a ventral pair of longitudinal muscles traversing 

 the prosoma from end to end (see Lankester, 7).^ With the 



' " The simple musculature in llie ancestor consisted of — (1) a pair of dorsal 

 longitudinal muscles passing from tergite to tergite of each successive seg- 

 ment ; (2) a similar series of paired longitudinal ventral muscles ; (3) a pair 

 of dorso-ventral muscles passing from tergite to stcrnite in each segment ; (1) 

 a set of muscles moving the coxa of eacii limb in its socket. The confluence 

 of the prostomium andtlie six anterior tergites to form a prosoniatic carapace, 

 as well as the specialisation of the six pairs of ap|)endages of tlie prosoma, 

 was common to the ancestors of both Limulus and Scorpio. This modi- 



