252 ANNELIDA. 



ovoid vesicle, or ampulla ; it then runs outwards, furnishing an ascend- 

 ing branch to the alimentary canal, and on arriving at the base of the 

 feet, or locomotive appendages, gives off several small anastomosing 

 branches, forming a sort of vascular network, whence vessels are supplied 

 to the corresponding branchial filaments. 



(657.) The blood, after being subjected to the influence of oxygen in 

 the branchial appendages, is returned by other transverse vessels which 

 run along the interannular septa to the alimentary canal, where they 

 ultimately discharge themselves into the large median trunks (Z) situ- 

 ated upon the dorsal aspect of the intestine. 



(658.) Considered generally*, it will be perceived that the distribu- 

 tion of the vascular trunks in the Eunices is pretty much the same as 

 that which exists in the Terebellaa ; but when their functions are con- 

 sidered, and the relations in which they stand relatively to the respi- 

 ratory apparatus, very important differences are at once apparent be- 

 tween the two genera. 



(659.) In Eunice, it will be seen (fig. 123) that Professor Milne-Ed- 

 wards has described and figured the branchial vessels as ampullated 

 soon after the origin of each from the common trunk, the ampulla? being 

 designed to fulfil the function of branchial hearts. These vessels there- 

 fore, according to the representations of Milne-Edwards, are, in Eunice, 

 the analogues of those remarkable cardiac vessels (pulmonary hearts) de- 

 scribed by Duges in the Leech. The existence of these latter vessels has 

 already been found to be extremely problematical ( 583). According 

 to the observations of Dr. Williams, these vessels in Eunice present 

 nothing approaching to the ampulla? figured in the illustrations of 

 Milne-Edwards. The pouched dilatations are produced by the dissec- 

 tion and exposure to atmospheric stimulus, just as in the Earthworm 

 the moniliform character of the descending vessel was shown to be 

 caused by the stretching. In Eunice the lateral large segmental branches 

 are relatively large at first, but soon divide into three lesser branches, of 



* The following axioms may be laid down relative to the circulation of the 

 blood: 



1st. In all Annelids, the blood flows in the great dorsal trunk from the tail towards 

 the head. 



2nd. In all Annelids, the blood flows in the great ventral trunk from the head 

 towards the tail. 



3rd. In the whole integumentary system of vessels, the blood moves from the great 

 ventral towards the great dorsal trunk : this movement constitutes the annular, or 

 transverse circulation. The main current of the blood in the ventral trunk pursues 

 a longitudinal course until exhausted by successive lateral deviations. 



4th. In Annelids, the intestinal system of vessels consists of four longitudinal 

 trunks : one dorsal, which may be called dorso-intestinal ; one ventral, which may 

 be distinguished as the sub-intestinal ; and two lateral. These several trunks are 

 joined together by circularly-disposed branches bearing a dense glandular capillary 

 system. In the inferior intestinal system the general movement of the blood is from 

 before backwards ; in the circular branches, from the ventral towards the dorsal trunk. 



