AND ENDOSKELETAL SYSTEMS OF SCORPIO. 307 
IX. Muscles to the Pericardium. (See Plate LX XVII. figs. 1, 4, 5, and 
Plate LX XX. 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 lamelle 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 nuscle (153) of the fourth segment of the mesosoma; the 
pericardio-ventral muscle (154) 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. Ietasoma. 
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 
