76 



The Larva of Chironomus 



The larva of Tanypus exemplifies this type, as also do 

 certain unnamed Chironomus-larvae. 



The second type of dorsal vessel is found in the larva 

 of Chironomus dorsal is. Here the heart and aorta are 

 clearly differentiated, the heart being much the wider of 

 the two ; it is furnished with two pairs of valvular inlets, 

 and its muscles, while still retaining something of the origi- 

 nal semicircular arrangement, form bundles of striated 

 fibres. The aorta has no muscle-cells, inlets, or valves, 

 except the pair at its base (which properly belong to the 

 heart) ; its wall, though very elastic, is not contractile. 



In a dorsal vessel of the first type the valves are usually 

 of very simple structure, and arise by modification of the 

 muscle-cells ; where inlets form, the adjacent muscle-cells 

 do not altogether lose their original character ; the more 

 complex aortic valves may be absent altogether. In the 

 other type the simple cellular valves almost or altogether 

 disappear, aortic valves of complex structure are found, 

 and the inlets themselves become valvular. Graber ^ has 

 described and figured the appearance which the posterior 

 inlets present, during contraction, in a Chironomus -larva 

 of this kind. The muscular bands adjacent to the inlet, 

 in which nuclei can often be distinguished, appear to 

 cross one another and to unite where they cross, forming 

 a figure-of-eight. During contraction, they appear to 

 close the vessel and the inlets by one operation. All the 

 valves found in the heart of any Chironomus, whether 

 cellular, ostial, or aortic, appear to be derived from the 

 semicircular muscle-cells. 



Sections 

 of dorsal 

 vessel. 



Course of 

 blood. 



Thin sections through the heart of a Chironomus-larva 

 show that there is an outer fibrous layer, an intermediate 

 space in which the muscle-cells lie, and an inner mem- 

 brane or endocardium. 



The blood passes from the heart to the aorta, and so to 

 the head, where it escapes into the body- cavity, bathing 

 all the viscera contained therein. A small portion of the 

 blood is distributed to the respiratory tubules, and 



dor.-al Vessel the heart does not extend backwards into the eleventh post- 

 cephalic segment. 



' 'Propulsat. Apparat d. Insekten.' tiitsb. rf. A". Akad. Wien, 1872, fig. 7 ; 

 •Javvorowski, 1879, ^S^- 24. 25. 



