HEART. 05 



system. While they are known chiefly in Cambrian and Lower Ordovician trilobites, there 

 is no evidence that the organs represented were not present in later forms, even if the shell 

 may not have been affected by them. While they indicate very fine, thread-like canals, the 

 present evidence seems to be in favor of assigning to them the function of lodging the glands 

 which secreted the principal digestive fluids. 



HEART. , 



Illcenus. 



Volborth (1863, pi. I, fig. 12 = our fig. 26) has described the only organ in a trilobite 

 which suggests a heart. A Russian specimen of Illcenus with the shell removed shows a 

 somewhat flattened, tubular, chambered organ extending from under the posterior end of 

 the cephalon to the anterior end of the pygidium. The posterior nine chambers were each 

 1.5 mm. long and 1.5 mm. wide, while the two anterior chambers were respectively 2.5 mm. 



Fig. 26. Copy of Vol- 

 borth's figure of the heart 

 of Illanus. 



Fig. 27. Heart 

 of Apus. Copied 

 from Gerstacker. 



and 3 mm. wide. These were all under the thorax, and at least two more chambers are 

 shown under the cephalon, but rather obscurely. The species of the Illcenus is not stated, 

 but since no Illcenus has more than ten segments in the thorax, and this tube has at least 

 thirteen chambers, it is evident that its constrictions are inherent in it, and are not due to 

 the segmentation of the thorax. Beecher has made a passing allusion to this organ as an 

 alimentary canal. This was the original opinion of Volborth. Pander, however, suggested 

 to him that it might be a heart. The alimentary canal of Cryptolithus does not show any 

 constrictions, while the heart of Apus (see fig. 27) and other branchiopods does show them. 

 It should be noted, further, that while this heart enlarges toward the front, it is everywhere 

 very small as compared with the width of the axial lobe, and much narrower than sections 

 of Ccraurus and Calymene would lead one to expect the alimentary canal of Illcenus to be. 

 Where the heart is 1.5 mm. to 3 mm. wide, the axial lobe is n mm. wide. 



While this may be merely a cast of the alimentary canal it is sufficiently like a heart to 

 deserve consideration as such an organ. 



Ceraurus and Calymene. 



Nothing suggesting a heart has been seen in the sections of Ceraurus and Calymene. 

 The mesenteron and its sheath crowd so closely against the dorsal test in the anterior part 



