walling: the acridid^an heart. 365 



influences for its control is shown by several experiments. As 

 was described in my previous paper, the heart will contract 

 many hours after being entirely isolated from all other sys- 

 tems. If all the blood is entirely removed from such an iso- 

 lated heart (fig. 1) by means of filter-paper, the contractions 

 soon cease. But if Ringer's solution is then placed upon the 

 heart, in the body-cavity, the contractions will be resumed. 

 This fluid seems to act in this respect as the animal's own 

 blood. The heart usually contracts at the same rate through- 

 out its whole extent, but this is not always the case. I have 

 observed difl'erent rates in different segments. 



An isolated heart may be sectioned at each abdominal seg- 

 ment, and if kept moist with Ringer's solution, each small 

 segment will contract for several hours. In this case the seg- 

 ments do not contract in unison. 



HISTOLOGICAL STRUCTURE OF THE HEART AND PERICARDIAL 



CELLS. 



The heart, as seen in cross-section, plate XXXVII, figure 2, 

 lies within the pericardial sinus, and on its dorsal aspect 

 is suspended to the chitin by muscle-fibers and connective 

 tissue. Fibers also lead off from the ventral side of its mus- 

 cular coat and extend out into the fibrous tissue (b, fig. 2), 

 and to the so-called alary muscles. These alary muscles are 

 arranged in fan-shaped bundles, as is shown in figure 1. The 

 wide part extends over the heart, to which it is attached, and 

 the narrow part, or handle of the fan, is attached to the tergite 

 of each segment. Some of the fibers of these muscles extend 

 from their attachment on one side of the wall entirely over to 

 their attachment on the opposite side, while others seem to end 

 in the heart-muscle proper. As viewed from the ventral side, 

 these muscles are seen to contract and expand slightly with the 

 pulsation of the heart, but they also have a pulsation independ- 

 ent of the heart's contraction, and usually at a much slower 

 rate. It is probable that these contractions help the blood to 

 pass through into the pericardial sinus. 



The muscle-fibers of the alary system are striated and con- 

 tain elongated nuclei. 



The muscular coat enclosing the heart-cavity is composed of 

 two kinds of cells — one consisting of long muscle-fibers ex- 

 tending around the tube and out into the surrounding tissue 

 as a sort of supporting structure (f, fig. 2, and b, fig. 7, plate 



2-Univ. Sci. Bull., Vol. IV, No. 17. 



