236 



THE VITAL FUNCTIONS. 



traced in them : an organization of this kind has 

 already been noticed in Tcenice.* 



It was, till very lately, the prevailing opinion 

 among naturalists that all true insects are nou- 

 rished by imbibition, and that there exists in 

 their system no real vascular circulation of 

 juices. In all the animals belonging to this 

 class, and in every stage of their developement, 

 there is found a tubular organ, called the dorsal 

 vessel, extending the whole length of the back, 

 and nearly of uniform diameter, except where it 

 tapers at the two ends. It contains a fluid, 

 which appears to be undulated backwards and 

 forwards, by means of contractions and dilata- 

 tions, occurring in succession in different parts 

 of the tube ; and it is also connected with 

 transverse ligamentary bands, apparently con- 

 taining muscular fibres, capable by their action 

 of producing, or at least of influencing these pul- 

 satory movements. An enlarged representation 

 of the dorsal vessel of the Melolontha vulgaris, 

 or common cockchaffer, isolated from its attach- 

 ments, is given in Fig. 333, showing the series 

 of dilatations (v, v, v) which it usually presents 

 in its course ; and in Fig. 334 the same vessel is 

 exhibited in connexion with the ligamentary and 



* Page 83, in this volume; Fig. 247. The family of Pla- 

 naricB present exceptions to this general rule : for many species 

 possess a system of circulating vessels. See Dug^s, Annales 

 des Sciences Naturelles; xv, 161. 



