184 ALEXANDER PETRUNKEVITCH 



The circulatory system of insects is somewhat better known 

 in this respect, yet here also the data are quite inadequate to 

 form a clear judgement. Popovici-Baznosanu has described the 

 heart in the Chironomus larva and states that the aortic valves 

 are situated close to the anterior end of the fifth segment. In 

 other larvae the heart had been described by other authors as 

 situated near the rear end of the body. In some larvae the heart 

 is not even situated directly under the dorsal body wall, but Hes 

 considerably deeper in the body cavity. The structiire of the 

 cardio-aortic valves, too, seems to be not only different from that 

 of the aortic valve in Arachnida, but not always of the same type 

 in all insects. Moreover, according to Zawarzin there are modified 

 ostia in the aorta itself. It is evident that the first step must be 

 in finding the true limits of the heart itself in insects. Mean- 

 while all conjectures would be entirely out of place. 



SUMMARY AND CONCLUSIONS 



1. In comparing the segmentation in arthropods the uncer- 

 tain method of counting somites beginning with the anterior 

 end should be abandoned. Instead, some structure should be 

 chosen which has permanent value for a number of forms within a 

 class and used as a starting-point of comparison. 



2. Such structure in the case of Arachnida is furnished by 

 the cardio-aortic valve which marks the division line between 

 the last thoracic and first abdominal segments. 



3. The method applied to Limulus leads to the conclusion 

 that the carapace of Limuhis is more complicated than in Arach- 

 nida, having two abdominal tergites drawn into the horseshoe- 

 shaped thoracic tergite with which they have fused anteriorly 

 and laterally. 



4. A further conclusion is that the midcorporal articulation in 

 Limulus is not between thorax and abdomen, but between the 

 second and third abdominal somites. 



5. The structure of the circulatory system in Arachnida fol- 

 lows a general plan given in the text. 



6. The number of postoral somites in adult Archnida is seven- 

 teen. Five of these are thoracic and twelve abdominal. 



