AND ENDOSKELETAL SYSTEMS OF SCOKPIO. 357 



IX. Muscles to the Pericardium. (See Plate LXXVII. figs. 1, 4, 5, and 

 Plate LXXX. fig. 15.) 

 There is a series of muscles attached to the ventral wall of the pericardium, running 

 ventrally and attached to the ventral portion of the investment of connective tissue 

 which lines the body ; they lie slightly anterior to the dorso-ventral muscles, mentioned 

 before as being attached to the tergites and sternites of the mesosoma. There is one 

 in each of the five last segments of the mesosoma, and two in the first segment of the 

 metasoma. Dorsally they are attached nearer to the median line than the dorso- 

 ventral muscles, which are attached to the tergites by the side of the pericardium ; but 

 before reaching the sternal surface they cross over, and their ventral attachment is 

 exterior to that of the dorso-ventral muscles. In the four segments in which the lung- 

 books occur they spread out ventrally, and are attached to the surface of the venous 

 sacs, into which the introverted lamellae of the branchial appendages (or lung-books) are 

 sunk. These muscles are hollow, both above and below ; but the middle region is solid, 

 so that the blood does not pass from the pulmonary sinus to the pericardium through 

 them, as might at first seem likely. They are exactly equivalent to the veno-pericardiac 

 muscles of Limulus (68), the " brides transparentes " of M. Alphonse Milne-Edwards. 



a. Mesosoma. 



The pericardio-ventral muscle (131) of the second segment of the mesosoma is 

 attached dorsally to the pericardium, running forward closely in front of the dorso- 

 suprapectinal chondrite muscle (66) ; it is attached ventrally to the connective investment 

 of the body exterior to the above-mentioned muscle. 



The pericardio-ventral muscle (132) of the third segment of the mesosoma is attached 

 dorsally to the pericardium, running down closely anterior to the dorso-ventral 

 muscle (67) ; it is attached ventrally to the first lung-sac, exterior to the dorso-ventral 

 muscle (67). 



The pericardio-ventral muscle (133) of the fourth segment of the mesosoma; the 

 pericardio-ventral muscle (134) of the fifth segment of the mesosoma; and the peri- 

 cardio-ventral muscle (135) of the sixth segment of the mesosoma are similar to the 

 pericardio-ventral muscle (132) of the third segment, and take a corresponding position 

 in the segments to which they belong. 



b. Metasoma. 



The anterior pericardio-ventral muscle (136) of the first segment of the metasoma 

 is attached dorsally to the pericardium, running closely anterior to the dorso-ventral 

 muscle (71) of the first segment of the metasoma, and is attached ventrally to the 

 connective investment of the body exterior to the muscle (71). 



The posterior pericardio-ventral muscle (137) of the first segment of the metasoma is 



