CHAP. 71. STEUCTUKE OF THE HEART. 43 



of Natural History," this affected hesitation of the 

 schoolmen cannot dissuade me from seeking an answer, 

 which indeed presents itself most naturally from Dar- 

 win's point of view. 



As not only the Tanaides (which reasons elsewhere 

 stated (vide supra) justify us in regarding as particularly 

 nearly related to the primitive Isopod) and the 

 Amphipoda, but also the Decapod Crustacea, possess a 

 heart with three pairs of fissures essentially in the 

 same position ; and as the same position of the heart 

 recurs (vide infra) even in the embryos of the Mantis- 

 Shrimps (Squilla), in which the heart of the adult 

 animal, and even, as I have elsewhere shown, that of 

 the larvae when still far from maturity, extends in the 

 form of a long tube with numerous openings far into 

 the abdomen, we must unhesitatingly regard the 

 heart of the Amphipoda as the primitive form of that 

 organ in the Edriophthalma. As, moreover, in these 

 animals the blood flows from the respiratory organs to 

 the heart without vessels, it is very easy to see how 

 advantageous it must be to them to have these organs 

 as much approximated as possible. We have reason to 

 regard as the primitive mode of respiration, that occur- 

 ring in Tanais (vide supra). Now, where, as in the 

 majority of the Isopoda, branchiae were developed upon 

 the abdomen, the position and structure of the heart 

 underwent a change, as it approached them more nearly, 

 but without the reproduction of a common plan for 

 these earlier modes of structure, either because this 

 transformation of the heart took place only after the 



