170 



ANNELIDA. 



sides (d), which are highly contractile. These 

 ' moniliform vessels' are placed in a situation 

 corresponding to the ovaries : they are directed 

 downwards and open into a ventral vessel (6), 

 which occupies the middle line of the inferior 

 aspect of the animal, following the same track 

 as the sub-nerval vessel, but situated less 

 superficially. Its parietes are contractile, and 

 it may be seen alternately dilating and con- 

 tracting simultaneously at every part along 

 the whole of its extent. The blood flows 

 into this ventral vessel from before backwards, 

 and leaves it to re-enter the dorsal vessel 

 by passing through the branches (e) which 

 ascend perpendicularly to join the latter, on 

 either side of the alimentary canal, which they 

 thus embrace, and to which they furnish a great 

 number of ramifications. The blood con- 

 tained in the sub-nerval vessel flows equally 

 from before backwards, and ascends to re-enter 

 the dorsal vessel by lateral channels (_/'), ana- 

 logous to the anastomosingvesselswhich we have 

 just described, but situated more superficially 

 than those. These superficial transverse or 

 dorso-ubdominal vessels, as they are termed by 

 M. Duges, severally receive a large branch 

 from their corresponding deep-seated dorso- 

 abdominal vessel, and distribute to the skin a 

 number of ramifications which appear to be 

 specially destined to bring the blood into 

 contact with the oxygen necessary for respi- 

 ration.* 



In the genus na'is the moniliform vessels, 

 which in the earthworm perform in some degree 

 the office of a composite heart, seem to be re- 

 placed by a single pair of wide veins, which are 

 contractile and analogous to a divided heart, and 

 both the superficial and deep-seated transverse 

 vessels by which the blood ascends to the 

 dorsal trunk seem to rise from one and the same 

 ventral trunk ; so that the circulatory appa- 

 ratus is more simple in these annehda than 

 in the earthworms. The same plan pervades the 

 sanguiferous system in the other setiferous an- 

 nelidans, in which the branchiae are distributed 

 throughout the entire length of the body ; but 

 when these organs are collected together at a 

 determinate point of the anterior extremity 

 of the body it is a little different. Thus in 

 the terebellae the ventral vessel is seen to 

 bifurcate and to form two lateral brandies 

 which have the form of an arch, and which, 

 after having passed over the sides of the 

 oesophagus, re-unite above that tube to form a 

 single trunk. This trunk reaches the anterior 

 extremity and gives origin to three pairs of 

 primary branches, which descend to the vesi- 

 cular receptacles at the base of the branchiae, 

 and distribute the blood to these organs. 



In the leech-tribe the vascular system, on 

 the contrary, is more complicated, for the san- 

 guiferous circle is composed of four longitu- 

 dinal trunks, and the branches which bring 

 them into communication with each other. 

 Of the four longitudinal vessels two occupy 

 the dorsal and ventral aspects of the mesial 



* Sec Duges, * Rechcrches sur les Annclides 

 abranches,' Annalcs dcs Sciences "Nat. t. xv. 



line, and two the sides of the body. The 

 dorsal and ventral trunks communicate toge- 

 ther by dorso-abdominal branches correspond- 

 ing to each segment of the body. The lateral 

 trunks also render to the dorsal trunk a series 

 of dorso-lateral branches, and, moreover, mu- 

 tually communicate by a series of abdomino- 

 lateral branches which glide transversely be- 

 neath the nervous chords. The dorsal and 

 ventral vessels are evidently analogous to those 

 which we have designated by the same names 

 in the earthworm and nais; and the lateral 

 vessels may be compared to the sub-nerval 

 trunk of the earthworm, except that, instead 

 of being single and situated in the mesial line, 

 they form a circle in which the blood undu- 

 lates sometimes in one direction, sometimes in 

 another, but always pursuing an opposite 

 course in the two canals. Lastly, in addition 

 to the above ' general circulation/ there is 

 observed in the leech-tribe something ana- 

 logous to the ' lesser circulation,' (Jig. 72) : 



Fig. 72. 



S-l 



this is effected in the branches (b, e) of the dorso- 

 lateral vessels (), which are for the purpose of 

 bringing the blood into contact with the aerated 

 water contained in the small membranous 

 vesicles (_/') situated at the sides of each seg- 

 ment of the animal, and opening externally 

 upon the inferior aspect.* 



Respiration. From what has been said of 

 the mechanism of the circulation in the annelida, 

 it will be seen that respiration must be effected 

 either in the vesicles above mentioned, or on the 

 surface of the body. Such in fact is the case; 

 the skin is in general the seat of that function ; 

 but in the greater number of instances, the 

 integument, instead of maintaining the same 

 texture throughout, and acting upon the air in 

 the same manner at every point of its extent, 

 presents at particular spots peculiar modi- 

 fications, and thus gives rise to special organs 

 of respiration called ' branchiae.' 



The branchiae of the annelida are almost 

 universally membranous appendages, highly 

 vascular, fixed to a certain number of the feet 

 of the animal, or inserted upon the back near 

 the base of these organs. 



In the nereida and some other congeneric 

 annelida, the appendages which are designated 

 branchiae, and which in fact seem to be in an 



* See Moquin Tandon, op. cit. Duges, op. cit. 



