No. I.] STUDIES ON LIMULUS. 127 



Around the heart is a large pericardial sinus (Text-figs. 

 2-4; Pis. VIII and IX, Figs. 3, 5, and 6, p.s), enclosed by a 

 membranous pericardium (/.). Upon the neural side this mem- 

 brane is well defined, stretching across the body, between the 

 heart and the intestine. At the sides it is attached, in the abdom- 

 inal region, to the entapophyses, to the bases of the branchial 

 cartilages, and to the haemal carapace between the successive 

 entapophyses. In the cephalothorax it is attached to the first 

 pair of entapophyses, and to the carapace outside of the origins 

 of the inter-tergal muscles. Posteriorly and anteriorly the peri- 

 cardium is continuous with the neural walls of the heart. 



Upon the haemal side of the pericardial space the pericardium, 

 as such, does not exist, or, at least, is indistinguishable from the 

 epidermis. A considerable amount of areolar tissue fills the 

 haemal side, and many of the interstices of the pericardial 

 sinus. 



Eight pairs of rather broad bands of connective tissue, the 

 alary muscles (PI. IX, Figs. 5 and 6, a.l.m.^-'^) of Van der 

 Hoeven, spring from the lateral edges of the heart, opposite 

 the ostia, and fuse at their distal ends with the pericardium, 

 forming a strong lateral support for the heart. Those in the 

 abdominal region enter the venous canals opening into the 

 pericardial sinus. 



Neurally the heart is attached to the pericardium throughout 

 its entire length by numerous connective-tissue fibers. Hae- 

 mally it is suspended from the carapace, opposite each pair of 

 ostia, by small strands of connective tissue which are continuous 

 with the longitudinal fibers of the heart. From the anterior 

 extremity of the heart, opposite the rudimentary ostia, a pair 

 of tendinous bands {al.m.^), comparable to a pair of alary mus- 

 cles, run forward and upward a short distance beyond the limits 

 of the pericardium, and attach themselves to the carapace close 

 to the insertions of the tergo-proplastral muscles (PI. IX, Fig. 

 5, t.p.m/). At the posterior end a sheet of connective tissue 

 attaches the extremity of the heart to the carapace. 



The pericardial sinus (Text-figs. 2-4; Pis. VIII and IX, 

 I^'i&s. 3, 5, and 6, p.s) surrounds the heart from the extreme 

 posterior end to a point about opposite the rudimentary ostia. 



