AND ENDOSKELETAL SYSTEMS OF LIMULUS. 325 



and runs slightly outwards and forwards to the uprising portion of the post-abdominal 

 sternite, to which it is attached. It is more laterally placed than no. 9, and below no. 7. 



No. 11. The Outer Sterno-pygal arises from the basal membrane of the spine more 

 laterally than no. 10. Passing below no. 8 it is attached to the floor of the metasoma. 



No. 61. A few muscular fibres, rising in the floor of the fourth mesosomatic segment, 

 run to the fifth and thence to the sixth segment, and end in the metasomatic sternite, 

 a good deal mixed with nos. 3 and 5. 



II. Borso-ventral Muscles. (Plate LXXV.) 

 No. 12. The Vertical Mesosomatic Mmcles. — Of these there are six on each side. 

 Each is inserted into one of the six mesosomatic (or abdominal) entochondrites on its 

 outer edge, and rising slightly obliquely across the bundles of no. 1, between the 

 branches of which it passes, is attached to the tergum anteriorly to the base of each of 

 the entapophyses, its general course being vertical. [It is especially noteworthy that 

 the first pair of this series passes from the entochondrite of the genital operculum to 

 the in-osomatic tergum, being attached just in front of the great entapophysis on each 

 side.— E. R. L.] 



No. 13. The Oblique Entapophysio-sternals. — A muscle from the second mesosomatic 

 entochondrite passes backwards, upwards, and outwards above no. 3, and below the 

 branches of no. 2, to its attachment to the deep (free) end of the fourth entapophysis. 



No. 14. A muscle, with a similar course, from the third mesosomatic entochondrite 

 to the fifth entapophysis. 



No. 15. A similar muscle from the fourth mesosomatic entochondrite to the sixtli 

 entapophysis. 



No. 16. A similar muscle from the fourth mesosomatic entochondrite to the metaso- 

 matic (seventh) entapophysis. 



No. 17. A similar muscle from the fifth abdominal entochondrite to the metasomatic 

 (seventh) entapophysis. 



No. 18. The Branchio-thoracic Muscles. — A series of muscles pass from the hollow 

 tendons which open at the stigmata of the mesosomatic appendages. There are six 

 pairs of these. Each tendon has attached to it a thickening bundle of muscle which 

 in its passage upwards and forwards passes outside no. 12 muscle to the inside of the 

 entapophyses, beneath the dorsal lateral p] astro-tergal muscle (52) to its attachment 

 to the carapace in the prosoma, alongside that of the coxo-tergal muscles, but nearer 

 the median plane. 



No. 19. From the bundle attached to the last stigma (see PI. LXXV. fig. 1), before 

 it has joined those arising from the anterior stigmata, there rises a muscle which 

 passes more directly upwards than the mass of the branchio-thoracic, and is attached 

 to the inner posterior face of the second entapophysis. 



