CIRCULATORY SYSTEM IN ARACHNIDA 181 



We may therefore form the following conclusions regarding 

 segmentation in scorpions and in Arachnida in general. The 

 body of an Arachnid is composed of twenty one somites, to wit : 

 1 the first ocular (median eyes in the scorpion, anterior median 

 eyes in the spider) ; 2 second ocular (lateral eyes in the scorpion, 

 eyes with inverted retina in spiders, i.e., anterior lateral, and the 

 four posterior ones) ; 3 rostral (upper lip) ; 4 cheliceral; 5 pedipal- 

 pal; 6 to 9 thoracic pedal; 10 to 21 abdominal. The first three 

 are originally preoral in position. The fourth or cheliceral 

 becomes preoral during development. The attachment of the 

 heart to the anterior edge of the dorsal wall of the first abdominal 

 somite and the formation at this place of the aortic valve 

 indicate the division line between the thorax and the abdomen. 

 The tenth somite is always rudimentary, having lost its identity 

 in all but its neuromere. The genital opening is on the eleventh 

 somite (second abdominal). A further fusion and ultimate loss 

 of the identity of somites in Arachnida involves the posterior end 

 of their body, beginning with the twenty-first somite and pro- 

 ceeding forward. In some cases, as in the eighteenth somite 

 in the scorpion, secondary or spurious segmentation may take 

 place, which has no relation to the original metamerism. If 

 there be more than three originally preoral somites, these would 

 have to be sought for in front of the first ocular somite. 



Turning once more our attention to Limulus, we may first 

 of all consider the homology of the thoracic and abdominal 

 somites established in a way excluding all doubt. The six pairs 

 of appendages belong to the same somites as in Arachnida, 

 the chilaria represent the tenth, the opercula the eleventh somite, 

 the five branchial neuromeres correspond to the twelfth to six- 

 teenth somites, and of the three postbranchial ganglia the last 

 is the result of fusion of the nineteenth to twenty-first neuromeres, 

 if the ancestor of Limulus possessed that many postbranchial 

 somites. 



The homology of the preoral somites is more troublesome. 

 Patten and Redenbaugh describe three preoral neuromeres, the 

 olfactory, median ocular, and lateral ocular. Shipley following 

 Carpenter recognizes only two somites, the median ocular and 



