VASCULAR CIRCULATION. 171 



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

 first rudiments of a vascular organization arc those observed 

 and described by I'iedemann, in the Jl^tcrivc, which are si- 

 tuated higher in the animal scale than McduscT; but whether 

 any actual circulation takes place in the channels constituted 

 by these vessels, which communicate both with the cavitv 

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

 determined with any certainty. The ITolothuriin, which 

 also belong to the order of Echinodcrmata,are furnisiied 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 Txnix:* 



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 enlaro-ed re- 

 presentation of the dorsal vessel of the Mclolontha vulgaris, 

 or common cockchaffer, 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, Tit^. 247. The family of rianarix present ex- 

 ccptions to this genenxl rule: for many species possess a system of circu- 

 lating vessels. Sec Duges, Amiales des Sciences Naturellesj xv. 161. 



