ORGANS OF CIRCULATION 397 



THE ORGANS OF CIRCULATION 



Although Malpighi was the first to discover the heart in the 

 young silkworm, it was not until 1826 that Carus proved that there 

 was a circulation of blood in insects, which he saw flowing along 

 each side of the body, and coursing through the wings, antennae, 

 and legs of the transparent larva of Ephemera, though three years 

 earlier Herold demonstrated that the dorsal vessel of an insect is a 

 true heart, pulsating and impelling a current of blood towards the 

 head. This discovery was extended by Straus-Durckheim, who dis- 

 covered the contractile and valvular structures of the heart. It is 

 noteworthy that both Cuvier and Dufour denied that any circula- 

 tion, except of air, existed in insects ; and so great an anatomist as 

 Lyonet doubted whether the dorsal vessel was a genuine heart, 

 though he pointed out the fact that there are no arteries and veins 

 connected with this vessel. Another French anatomist, Marcel de 

 Serres, thought that the dorsal vessel was merely the secreting 

 organ of the fat-body. 



The so-called peritracheal circulation claimed by Blanchard and by Agassiz has 

 been shown by McLeod to be an anatomical impossibility, the view having 

 first been refuted by Joly in 1849. 



Except the aorta-like continuation in the thorax and head which divides into 

 two short branches, there are, with slight exceptions (p. 405), no distinct 

 arteries, such as are to be found in the lobster and other Crustacea, and no 

 great collective veins, such as exist in Crustacea and in Limulus. This is 

 probably the result of a reduction by disuse in the circulatory system, since in 

 myriopods (Julidse and Scolopendridae) lateral arteries are said to diverge near 

 the ostia. 



a. The heart 



The heart or " dorsal vessel " is a delicate, pulsating tube, situated 

 just under the integument of the back, in the median line of the 

 body, and above the digestive canal. It can be partially seen with- 

 out dissection in caterpillars. It is covered externally and lined 

 within by membranes which are probably elastic ; and between these 

 two membranes extends a system of delicate muscular fibres, which 

 generally have a circular course, but sometimes cross each other. 

 The heart is divided by constrictions into chambers, separated by 

 valvular folds. The internal lining membrane referred to forms the 

 valvular folds separating the chambers. Each of these chambers 

 has, at the anterior end, on each side, a valvular orifice (Fig. 370, 

 ostiurn, i) which can be inwardly closed. 



