126 PATTEN AND REDENBAUGH. [Vol. XVI. 



a large portion of the cephalothorax and abdomen. These com- 

 municate with ducts which are collected into the two pairs of 

 large hepatic ducts [h.d."-'^) entering the intestine, 



5. The Circulatory System. 



The circulatory system has been worked out very accurately 

 by A. Milne-Edwards, but some structures have been over- 

 looked in connection with the heart. This organ will, therefore, 

 be taken up in some detail. 



a. The Heart (Text-figs. 2-4 ; Pis. VIII and IX, 

 Figs. 3, 5-8, ht). 



The heart, which is very large in Limulus in comparison 

 with the size of the body, lies on the haemal side of the 

 animal, directly beneath the carapace and haemal to the 

 intestine. It extends from a point midway between the lateral 

 eyes back to about the middle of the abdomen, being fully one- 

 half as long as the body exclusive of the caudal spine. It has 

 the general appearance of a jointed tube, and attains a length 

 of about five inches in the adult male and about six inches 

 in the female, with a diameter of from half to three-quarters 

 of an inch. Longitudinal strands of connective tissue give it 

 a striated appearance, and a large median ganglionated nerve 

 (Text-figs. 3 and 4; Pis. VIII-X, Figs. 3, 5, 6, 8-10, in.c.7i.) 

 is very conspicuous upon the haemal side. In cross-section it 

 is somewhat triangular in the middle portion, but is flattened 

 haemo-neurally toward the extremities. The largest portion is 

 just back of the middle, and from this point it tapers in both 

 directions. 



There are eight pairs of transverse slit-like ostia (Pis, VIII— 

 X, Figs, 3, 5, 6, 8, and 9, os.^'^) upon the haemal side of the 

 heart, partially concealed by a grating of longitudinal connec- 

 tive-tissue strands lying across the openings. A ninth pair of 

 rudimentary ostia (PI. IX, Fig. 8, r.os.) are discernible at the 

 anterior extremity. They appear as two shallow pits on the 

 inner surface of the haemal wall of the heart, just behind 

 the aortic 7irc\\c^{ao.a.) and in front of the aortic valve {a.v). 



