182 THE VITAL FUNCTIONS. 



it in the contrary direction, and complete the circuit of its 

 course. The ramifications and lateral connexions of the 

 minuter branches are often so numerous as to compose a 

 vascular net-work, covering a considerable extent of surface. 

 This general description of the circulatory system is appli- 

 cable to the tribes of Annelida possessing the simplest struc- 

 ture, such as the Nais, the Nereis, and the Leech; genera 

 which include a great variety of species of different shapes 

 and sizes. 



Although the vessels themselves may be plainly discerned, 

 It is not so easy to determine the real course which the blood 

 takes while circulating within them; and we accordingly 

 find very great discordance in the reports of different phy- 

 siologists on this subject. De Blainville asserts that in all 

 the Annelida, the blood in the dorsal vessel is carried back- 

 wards, that is, from the head to the tail; a motion, which, of 

 Course, implies its return in the contrary direction, either in. 

 the lateral or the abdominal vessels. In the Nais, the Nereis, 

 and the Leech, these last vessels are two in number, situated 

 at the sides of the abdominal surface of the body. Carus 

 adds his testimony in favour of this mode of considering the 

 circulation in the Annelida. On the other hand, Spix, Bon- 

 net, Sir Everard Home, and Duges, describe the course of 

 the blood as quite the opposite of this, and maintain that it 

 moves backwards, or towards the tail, in the abdominal ves- 

 sels; and forwards, or towards the head, in the dorsal vessel. 

 Morren, who is the latest authority on this subject, gives 

 his testimony in favour of the latter view of the subject, as 

 far as relates to the dorsal vessel of the Erpobdella vulga- 

 ris* an animal allied to the Leech, and already noticed in 

 the account of the mechanical functions of this tribe :t but he 

 considers the abdominal vessel as performing also the same 

 function of carrying the blood forwards towards the head, 

 and the two lateral vessels as conveying it backwards, thus 

 completing the circuit. This is illustrated by the diagram 



* Hirudo vulgaris. (Linn.) Nephelis vulgaris. (Savigny.) . 



f Vol. i. p. 195, where a delineation of this animal was given, Fig. 130. 



