AND ENDOSKELETAL SYSTEMS OF LIMULUS. 331 



Fifth : No. 5 is continued. 



No. 12 is present as the fifth pair of vertical mesosomatic muscles. 



No. 17, a fifth pair of oblique entapophysio-sternal muscles, arising (like 



No. 16) from the seventh pair of entapophyses, is inserted into this fifth 



mesosomatic entochondrite. 

 No. 63 arising from the second mesosomatic entochondrite is inserted here. 

 No. 64 arising from the third mesosomatic entochondrite is inserted here. 

 Sixth : No. 5 is continued from the preceding entochondrite to this one, and passes 



on from this to its final insertion in the solid metasomatic sternite (epi- 



dermal chitin forming the floor of the hind part of the meso-metasomatic 



carapace). 

 No. 12 is present as the sixth and last pair of vertical mesosomatic muscles. 



VI. Pharyngeal. 



No. 66. The SternO'pharyngeal Muscles. — From the ventral surface of the anterior 

 curvature of the alimentary canal a number of muscular bands pass, though not in 

 regular distinct bands, to the subfrontal area of the chitinous prosomatic carapace. 



No. 67. The Plastro-huccal Muscle. — A few muscular fibres go from the oesophagus 

 to the ventral surface of the entochondrite. 



VII. Pericardiac. 



No. 68. The Veno-pericardiac Mu.icles. — On the fioor of the mesosoma (as in the 

 Scorpion), on each side, near the middle line, is a blood-sinus, the " venous collecting- 

 sinus " as M. Milne-Edwards calls it ; here the blood collects from the body, on its way 

 to be aerated in the leaves of the gill-books. From the floor of the sinus a vessel goes 

 into each of the abdominal appendages ; from the space between the anterior and 

 posterior lamellae of these it reaches the gill-books. 



From the roof of this collecting-sinus in each abdominal segment a muscle arises, 

 which is inserted into the floor of the pericardium ; it is a narrow, flat, almost trans- 

 parent mass of muscular fibre, called by M. Milne- Edwards "brides transparentes," and 

 not recognized by him as composed of muscular tissue. These are the veno-^Jericardiac 

 muscles. Besides the six pairs in the abdomen, two pairs occur on each side of the 

 thoracic entochondrite (PI. LXXV. fig. 2). 



DESCRIPTION OF PLATES LXXII. to LXXVI. (illustrating Limulus). 



References. 



a. Cliitmous sclerite on the anterior plate ah. Anterior border of an eutocoxite. 



of a mesosomatic apiiendage. A.c.en. Anterior cornu of the entosternite or 

 Ah. Anterior thickened border of the meso- plastron : the proplastral process, 



metasomatic carapace. Al. Alimentary canal. 



