488 



If the transformation of the heart of Nasonia is any indi- 

 cation of what happens in the blow-fly, then there can be no 

 doubt that Weismann's view was the more correct. In the 

 heart of Nasonia a metamorphosis occurs quite as profound as 

 that observed in any of the other organs. 



The Structure of the Larval Heart. 



The heart of the larva (fig. 211) is a long tube, running 

 right along the mid-dorsal region of the body and gradually 

 bending downwards near the middle of the body, terminating 

 in front, close behind the brain. It measures about 1'6 mm. 

 in length. It is widest behind, where it measures '08 mm. in 

 diameter and gradually tapers in front into a long capillary 

 tube — the ''aorta." 



The heart lies within the pericardium, a tube composed 

 of a fine delicate membrane (fig. 213), which opens just behind 

 the posterior part of the heart by a large funnel-shaped open- 

 ing (figs. 211, 212), and tapers gra/dually anteriorly, eventu- 

 ally fusing in the anterior part of the body, with the heart. 

 y The pericardial walls are composed of a single layer of greatly 

 ^ flattened cells (fig. 213), but are quite devoid of muscles. 



While the pericardium has a wide funnel-shaped opening 

 into the body cavity behind, the heart itself is closed, and has 

 no communication with the pericardial cavity except by a 

 series of six pairs of minute openings, the ostia. These ostia 

 are usually very difficult to observe, the only prominent ones 

 being a single pair at the posterior end of the heart (fig. 212). 

 The cardiac walls are here deflected inwards to form a valve, 

 which allows blood to flow only from the pericardial cavity 

 into the heart. 



Neither the walls of tlie pericardium nor of the heart 

 are themselves contractile; pulsation of the heart is produced 

 by the contracting of certain very delicate muscles inserted in 

 the walls of the heart in irregular pairs at intervals along its 

 length; the other ends of these minute muscles are inserted 

 directly on to the dorsal integument. The pericardial walls 

 are drawn out into long conical processes, within which these 

 muscles lie (fig. 211). So far as I could observe, the peri- 

 cardium is itself not contractile, and alary muscles appear to 

 be quite absent. Tlie pericardium, then, appears to be different 

 from what is supposed usually to occur in insects. 



The cells of the heart undergo the same changes during 

 larval life as do those of the other specialized larval organs; 

 there is a great increase in cell size, with a total absence of 

 cell division. In the adult larva they have attained a great 

 size (fig. 216) ; they are very flat and measure as much as 



