THE ORGANS OF DIGESTION. 155 



be but one aperture in Phasma, which also has but one pair of lateral 

 muscles. By means of these apertures the heart is divided into so 

 many chambers, for behind each opening there are valves which separate 

 the preceding space from that behind the opening, so that in Melo- 

 lontha there are eight (PI. XXI. f. 1 8.) such consecutive chambers. 

 The first, which lies close to the dorsal sheath of the last abdominal 

 segment, is the smallest, and consists of one heart-shaped bag, which 

 in front, towards the head, has an opening like a slit. The lips of this 

 aperture consequently form the anterior side of the bag and close it, if 

 blood, pressing forward from within, does not part them. The blood 

 enters it through two small apertures, which likewise lie in front upon 

 each side of the bag, but it cannot flow back through the same openings, 

 for a half-moon-shaped valve which is affixed within the cavity of the 

 bag beneath the aperture closes upon it, and thus, when the heart con- 

 tracts, the blood must necessarily pass through the anterior opening. 

 This first and most posterior chamber of the heart is succeeded by 

 another in front, formed very similarly, but longer and more cylindrical, 

 and which has also an aperture behind, viz. the anterior one of the first 

 chamber. It is through this that the blood passes from the first cham- 

 ber to the second when the heart contracts, and upon its dilatation 

 blood pours into the chambers through the two lateral anterior open- 

 ings. Thus, therefore, each chamber is always provided with blood, 

 for the blood streams from one chamber to the other, beginning at the 

 posterior, when that which has been received through the lateral open- 

 ings from the cavity of the abdomen passes on by their successive con- 

 tractions. We will explain how this contraction (systole) and dilata- 

 tion (diastole ) of the heart take place after we have said a few words 

 upon its structure. 



118. 



According to Straus, two membranes are observed in the heart, the- 

 exterior of which is smooth, dense, and longitudinally fibrous, conse- 

 quently muscular. It is this which forms the above-described valves, 

 for at the two margins of each lateral aperture it bends inwards. The 

 posterior return forms the inner valve of that opening, and the anterior 

 return the partition of the chamber, or both the anterior ones form the 

 lips of the anterior opening. Both valves, as well as the entire internal 

 lining of the heart, are covered with a transversely folded and looser 



