362 PROF. E. R. LANKESTER ON THE MUSCULAR AJSD 



segment ; 2, a similar series of paired longitudinal ventral muscles ; 3, a pair of dorso- 

 veutral muscles passing from tergite to sternite in each segment, possibly derived from 

 the circular muscular layer external to the longitudinal which the soft-bodied fore- 

 runners of these chitinized Arthropods must have possessed ; 4, a set of muscles moving 

 the coxa of each limb in its socket, and consisting of probably an anterior and posterior 

 dorso-coxal from the tergite of each segment to the coxa of each of its two limbs, and 

 a similar anterior and posterior stemo-coxal muscle passing from the sternite of each 

 segment to the coxa of each of its two limbs ; 5, a pair of muscles in each segment 

 passing from the floor of the great dorsal blood-sinus (the pericardium) to the roof of 

 the great ventral blood-sinus which expanded on each side over the bases of the appen- 

 dages (veno-pericardiacs) ; 6, muscular bands radiating from the wall of the pharynx 

 to the adjacent tergite and sternite. 



The departure of both Limulus and Scorpio from such an ancestral plan is probably 

 to be traced for a certain distance along a common path. 



The confluence of the prostomium and the six anterior tergites to form a prosomatic 

 carapace, and the suppression of appendages on the six segments of the metasoma, as 

 well as the specialization of the six pairs of appendages of the ])rosoma, as elongate 

 Hnbulatory and grasping organs, and of the six pairs of appendages of the mesosoma 

 •as' flattened natatory and branchial organs, was common to the ancestors of both 

 Limulus and Scorjno. This modification of form and siiecialization of body-regions 

 entailed a corresponding modification of the muscular system. The dorsal and ventral 

 longitudinal muscles of the prosoma were suppressed, as well as those of the aborted 

 metasomatic appendages. The muscles of the prosomatic limbs acquired larger size, and 

 became subdivided, whilst those of the mcsosomatic limbs retained their simplicity. 



Coming to the actual form of the living Scorpions and King Crabs, we find that the 

 former has retained the separate chitinized segments of the mesosoma and metasoma, 

 whilst it has modified the four posterior pairs of mesosomatic appendages so that they 

 are no longer movable ; accordingly the Scorpions retain both the dorsal and ventral 

 longitudinal muscles of the mesosoma, whilst the muscles of the four posterior pairs of 

 mesosomatic appendages (the lung-books) are aborted. Limulus, on the other hand, 

 has retained both its prosomatic and mesosomatic appendages in full locomotor activity, 

 and not only are the hypothetical ancestral muscles of the latter appendages present, 

 but additional and very powerful muscles (the thoraco-branchials) have been developed ; 

 at the same time Limulus has undergone a very peculiar modification of the tergites of 

 the mesosoma and metasoma resulting in tlie formation of what is called " the abdo- 

 minal carapace." Consequently the dorsal longitudinal muscles are entirely suppressed, 

 with the exception of the powerful hinge-muscle (connecting the anterior and the 

 posterior carapace) and the interentapophysial muscles, which all arise from the pair of 

 great entapophyses of the prosoma, and are inserted into entapophyses of the mesosoma, 

 and may be regarded as modifications of the dorsal longitudinal intersegmental muscles. 



