VASCULAR CIRCULATION. 171 



and, in general, these vessels form a complete circuit. The 

 first rudiments of a vascular organization are those observed 

 and described by Tiedemann, in the *flsteriae, which are si- 

 tuated higher in the animal scale than Medusa?; but whether 

 any actual circulation takes place in the channels constituted 

 by these vessels, which communicate both with the cavity 

 of the intestine, and with the respiratory organs, is not yet 

 determined with any certainty. The Holothuriae, which 

 also belong to the order of Echinodermata,are furnished with 

 a complex apparatus of vessels, of which the exact functions 

 are still unknown. In those species of Entozoa which ex- 

 hibit a vascular structure, the canals appear rather to be ra- 

 mifications of the intestinal tube, than proper vessels, for no 

 distinct circulation can be traced in them: an organization of 

 this kind has already been noticed in the Tsenise* 



It was, till very lately, the prevailing opinion among na- 

 turalists that all true insects are nourished by imbibition, 

 and that there exists in their system no real vascular circu- 

 lation of juices. In all the animals belonging to this class, 

 and in every stage of their development, 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 dilatations, occurring in succession in 

 different parts of the tube; and it is also connected with 

 transverse ligamentary bands, apparently containing muscu- 

 lar fibres, capable, by their action, of producing, or, at least, 

 of influencing these pulsatory movements. An enlarged re- 

 presentation of the dorsal vessel of the Melolontha vulgaris, 

 or common cockchaflfer, isolated from its attachments, 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 



* Page 64, in this volume, Fig. 247. The family of Planarise present ex- 

 ceptions to this general rule: for many species possess a system of circu- 

 lating vessels. See Duges, Annales des Sciences Naturelles; xv. 161. 



