476 



COMPARATIVE ANATOMY 



CHAP. 



O 



This expansion, however, seems to be merely a result of the elasticity 



of its own walls. The whole mechanism, however, requires fresh 



investigation. 



The heart ends blindly behind, but is continued anteriorly into an 



aorta, which empties the blood into the lacunar system of the body. 

 We can occasionally distinguish with special 

 clearness a ventral sinus surrounding the ventral 

 chord (in Myriapoda, Orthoptera, and perhaps also 

 in certain Apterygota) in this sinus the blood 

 flows from before backward. A nerve supplying 

 the heart like that in Peripatus has here and 

 there been observed. 



Myriapoda (Fig. 336). The heart runs 

 through the whole body, and has as many 

 chambers and pairs of alary muscles as there 

 are trunk segments. From the most anterior 

 chamber an aorta arises, which divides into three 

 branches in the head. In the lulidce and 

 Scolopendridce lateral arteries are said to diverge 

 in the neighbourhood of the ostia. 



Hexapoda, Apterygota. The heart of the 

 Tliysanura consists of 9 chambers, and has 9 

 pairs of ostia and 9 pairs of weakly developed 

 alary muscles ; this is the highest number reached 

 among the Hexapoda. It runs forward as far as 

 the last thoracic segment, or even to the second ; 

 this is an important fact, as in all Pterygota, 

 with the exception of a few Orthoptem (Blatta), 

 the heart is exclusively confined to the abdomen. 

 The aorta is short in the Thysanura, as the heart 

 extends so far forward. The number of cardial 

 chambers and pairs of ostia is reduced in the 



Collembola (5 in Macrotoma). 



Pterygota. With the exception of a few Orthoptera, the heart is 



restricted to the abdomen, and has 8 pairs of ostia at the most. The 



number of pairs of ostia, of alary muscles, and of cardial chambers 



may be reduced like the number of the ganglia of the ventral chord. 



The aorta runs through the thorax and may be traced as far as into 



the head. 



In the Lepidoptera, the aorta bends up dorsally in the thorax, widens considerably 

 and forms a loop (Fig. 348, ac, p. 488). In a few cases the aorta has been found 

 branched in the head. In the larva of the Ephe,meridce, blood vessels with walls of 

 their own pass from the last cardial chamber into the three caudal setse. The valves 

 between the last cardial chamber and the one before it are here placed in such a way 

 that they prevent the passage of the blood from the former into the latter. When 

 the last chamber contracts, therefore, the blood is driven into the three arteries of 

 the caudal setfe. 



FIG. 336. Anterior end 

 of the heart of Scolopen- 

 dra (after Newport), ac, 

 Arteria cephalica ; ab, art- 

 erial rings ; al, lateral art- 

 eries ; hk, cardial chamber ; 

 o, ostia of the heart; fm, 

 alary muscles of the same. 



