Chap. III. GENUS PLEUROBRACHIA. 227 



there were not this natural disposition in all locomotion to influence the j^rocess 

 of respiration more than any other system, why should not the blood, when such 

 powerful motions take place, be accumulated in any other part of the body, — 

 for instance, in the tail, which is the very cause of the motion, — rather than in 

 the gills ? In Birds, the extensive development of the lungs, and the iDrolongation 

 of air sacs into the abdominal cavity, the wings, and the sternum, in those most 

 remarkable for their power of flight, plainly indicate again the most strict con- 

 nection between locomotion and resj^iration, though the nature of this connection 

 is perhaps different from that observed in the lower classes. Nevertheless, it exists 

 even in these, and can be traced to a very remarkable extent. We cannot fail 

 to trace similar relations among Mammalia also, though here the influence between 

 the two functions is not so direct. However, it must be acknowledged that it 

 is important enough, when we consider how the aquatic types have to accommo- 

 date all their movements to the wants of the system for atmospheric air, and 

 remain constantly within reach of the surface, in order to be able to return to 

 it in a short time. How much the breathing is affected by violent movements 

 is so well known to every one, that the existence of accessory muscles of respi- 

 ration in Mammalia, the antagonism between the pectoral and abdominal muscles 

 and the diaijhragm, and the use of belts by athletes in running, leaping, or wrest- 

 ling, need only to be mentioned as evidence of this mutual relation. Of course, 

 in animals in Avhich all the functions have reached a great degree of independence, 

 they are no longer subservient to each other to such a degree as they are in 

 the lower types ; but even the unpleasant influence which excessive exercise of 

 the locomotive powers has upon respiration in the higher animals, shows the inti- 

 mate relation which prevails in the plan of organization. 



It has already been mentioned, that there is a wide chymiferous cavity in the 

 centre of the l^ody of this animal, trending in the vertical direction of the digestive 

 cavity ; but the natural relations of these parts are so difficult to appreciate, the 

 ramifications of the chymiferous tubes so comphcated and nevei'theless so regular, 

 and again so movable in their constant contractions and dilatations, that, with all the 

 assistance of numerous drawings as given in my paper in the Memoirs of the 

 American Academy, I hardly expect to be able to give a correct idea of this 

 apparatus, unless the reader is willing to consider attentively every point of the 

 following description by itself, and to keep at the same time constantly in mind 

 the relative connection of all parts, and their bearing upon the general appearance 

 of the body. 



In the first place, let it be remembered that the central ch}'miferous cavity 

 and its main trunks undergo constant changes as to their size and outlines, ac- 

 cording to their temporary state of contraction and dilatation, and that both halves 



