CIRCULATORY SYSTEM IN ARACHNIDA 175 



tagma or mesosoma. Following this idea, he finds the same 

 tagmata in Limulus, the mesosoma being represented by the 

 genital opercula and the five gill-plates, while the metasoma is 

 reduced to a very small area around the anus, including the last 

 pair of lateral spines. 



Patten and Redenbaugh do not attempt to change the inter- 

 pretation of Lankester, but correct it in regard to the chilaria. 

 The presence of a distinct neuromere for this pair of appendages 

 having been established by Kingsley, our authors naturally 

 ascribe to them the value of a distinct metamere and consider 

 the chilarial somite as belonging to the cephalothorax. For 

 them, as for all previous investigators, the articulation between 

 the carapace and the abdomen is the segmentation line separat- 

 ing the cephalothorax from the abdomen. 



It would be useless to describe here in detail the entire cir- 

 culatory system of Limulus, but certain features of it must be 

 considered. The heart occupies the same position as in other 

 arthropods and extends from about the middle of the line passing 

 through the side eyes back to about the middle of the abdomen. 

 The heart has no opening posteriorly and the superior abdominal 

 artery is connected with the heart only indirectly through the 

 collateral arteries. Therefore, though occupying the same posi- 

 tion as the posterior aorta of Arachnida, the superior abdominal 

 artery of Limulus cannot be regarded as homologous with the 

 latter. The number of ostia is greater than in the scorpion, 

 inasmuch as Limulus has eight pairs. Patten and Redenbaugh 

 describe and figure a pair of rudimentary ostia in front of the 

 aortic valve. These may be the last remnants reminiscent of 

 a still older time when the ancestor had a heart extending farther 

 forward. The aortic valve has almost the same structure as in 

 Arachnida. In front of the valve "a, pair of tendinous bands, 

 comparable to a pair of alary muscles, 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" (p. 127). I may add that this connection 

 is so strong that in removing the carapace the heart is easily 

 injured, unless particular care is given to sever the connection of 



