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TEXT-BOOK OF ENTOMOLOGY 



Miall and Denny thus describe the different layers of the wall of the heart of 

 the cockroach : 



" There are : (1) a transparent, structureless intima, only visible when thrown 

 into folds ; ( 1 J) a partial endocardium, of scattered, nucleated cells, which passes 

 into the interventrieular valves ; (3) a muscular layer, consisting of close-set, 

 annular, and distant, longitudinal fibres. The annular muscles are slightly 

 interrupted at regular and frequent intervals, and are imperfectly joined along 

 the middle line above and below, so as to indicate (what has been independently 

 proved) that the heart arises as two half-tubes, which afterwards join along the 

 middle. Elongate nuclei are to be seen here and there among the muscles. Tlie 

 adventitia (4), or connective tissue layer, is but slightly developed in the adult 

 cockroach." 



Graber says that the heart of insects may be regarded not as an 

 organ de novo, but only as the somewhat modified contractile dorsal 

 vessel of the annelids, in which, however, the transverse arteries aris- 

 ing on each side became, with the gradual development of the tracheae, 



Fir,. 370. Part of the heart of Lucanun cn-riix : <i, the posterior chambers (the anterior ones 

 are covered by a part of the ligaments which hold the heart in place) ; i, auriculo-ventricular open- 

 ings ; <j, (j, the lateral muscles fixed by the prolongations A, h, to the upper side of the abdomen. 

 After Straus-Durckheim. 



superfluous and finally abortive. He describes it as a muscular 

 tube composed of very delicate annular fibres, which within and 

 without is covered by a relatively homogeneous, strong, elastic 

 membrane. 



The division into separate chambers is effected by means of a folding inwards 

 and forwards of the entire muscular wall. "A portion of each side of the 

 heart is first extended inwards so as very nearly to meet a corresponding portion 

 from the opposite side, and then, being reflected backwards, forms, according to 

 Straus (Consid., etc., p. 356), the interventrieular valve which separates each 

 chamber from that which follows it. Posteriorly to this valve, at the anterior 

 part of each chamber, is a transverse opening or .slit (Fig. 371, ft), the auriculo- 

 ventricular orifice, through which the blood passes into each chamber, and 

 immediately behind it is a second, but much smaller, semilunar valve (c), which, 



