222 



ANNELIDA. 



Fig. 108. 



other part of the body, while the primary lateral trunks are provided 

 with remarkable muscular parietes, their fibres being of the striped 

 kind. The fascicle of the muscle composing the walls is arranged in a 

 manner which is quite distinctive of, and peculiar to, this vessel ; it is 

 coiled with so much regularity as to enclose a perfect cylinder, in which 

 the blood flows : the longitudinal fibres are all suppressed, and therefore 

 the circular fascicles, lined within by a hyaline membrane, constitute 

 exclusively the coat of the vessel. Such a vessel is almost unique in . 

 structure in the animal 

 series; but none other would 

 perform so admirably the 

 peculiar duties for which it 

 is introduced into this part, 

 obviously as a special pro- 

 vision. Its business is to 

 transmit with augmented 

 force a current of blood in a 

 transverse direction from 

 the side to the ovario-ute- 

 rine organs (fig. 108, //), 

 which form a double longi- 

 tudinal series, one on either 

 side of the ventral mesial 

 line in each annular seg- 

 ment of the body. An ex- 

 press branchfrom the latero- 

 abdominal trunk on either 

 side is furnished to these 

 reproductive organs (fig. 

 108, 7i h)', so that the 

 amount of blood propelled 

 by this vessel, measured in 

 its totality, must be very 

 considerable, and the quan- 

 tity during the generative 

 season must undergo great 



increase in consequence of the augmentation of size which at that 

 period these organs experience. The lateral longitudinal vessel is 

 strikingly adapted to meet such alternation of extremes : constructed 

 of muscle, it readily yields, under the flow of the blood-tide to the 

 organs to whose wants it ministers, and its parietes augment by accele- 

 rated nutrition during the periods of increased local determination of 

 blood : formed of any other structure than muscle, such admirable 

 adaptive alternations could not happen. 



(581.) It has been generally considered that, in the abranchiate An- 



Diagram illustrative of the circulatory apparatus in 

 the Leech (Hirudo medicinal is). (After Dr. Williams.) 

 , great dorsal vessel; c, ventral vessel; dd, intercom- 

 municating vessels between dorsal and ventral trunks ; 

 ee, lateral abdominal trunks; ///, ovario-uterine 

 organs; g, vessels distributed over the csecal append- 

 ages to the stomach ; h h, branches from the lateral 

 abdominal trunks supplying the ovario-uterine appa- 

 ratus. 



