CHAPTER XII. 

 THE HEART. 



I. Location of The Heart. 



In the .annelids and in some primitive arthropods, the heart is a straight 

 tube lying on the opposite side of the body from the nerve cord, and extending 

 practically from one end of the body to the other. 



In the typical arachnids and in many of the higher arthropods (insects and 

 Crustacea), the primitive heart tube is shortened, in part by the conversion of the 

 anterior end into a non-contractile aorta, and in part by the absence of the more 

 posterior portion. The part that persists as a true pulsating heart is located, as 

 a rule, in the first eight post-thoracic segments, that is in the vagal and branchial 

 segments (Limulus and scorpion). (Fig. 3.) The heart may, in the earlier em- 

 bryonic stages, extend into the sixth (scorpion) (Figs. 15 and 16, h^), or into the 

 fifth and sixth thoracic segments (Limulus). (Figs. 141 to 151.) But these more 

 anterior heart segments are less highly developed, and may be reduced to a non- 

 pulsating chamber that forms the proximal end of the aorta. The most volum- 

 inous part of the heart in the adult Limulus is its posterior part, opposite the middle 

 branchial appendages, i.e., between the third and seventh pair of ostia. (Fig. i, B.) 



The location of the heart is greatly influenced by, or itself controls, the loca- 

 tion of the tracheal stigmata, the lung books, and the gills, since all these organs 

 retreat from the anterior head region in nearly the same order, and usually occupy 

 about the same post-thoracic segments. (Fig. 3.) 



The gradual retreat of the heart and the respiratory organs from the head and 

 thorax, and their concentration into a special group of post-thoracic segments 

 may be readily followed to its culmination in such forms as Limulus, scorpion, 

 and spiders. This striking process becomes especially significant when it is seen 

 that in the vertebrates also these organs occupy, as nearly as one may determine, 

 the same metameres. (Fig. 3, D, 308.) 



II. Development of The Heart. 



The location of the heart is determined by very remote but persistent condi- 

 tions that affect the form and structure of the whole anterior part of the head and 

 trunk. The principal event in these changes, which have been and are pro- 

 gressive, affecting the embryos of all segmented animals alike but in a varying 

 degree, is the gradual disappearance from before backward of the segmented 

 lateral plates of mesoderm belonging to the head and thoracic metameres. 



In Limulus, in the scorpion, and in spiders, the surviving mesoderm of the 

 cephalothorax consists almost exclusively of the six pairs of thoracic somites (head 



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