206 INVERTEBRATE MORPHOLOGY. 



and there in the dissepiments, and in some forms, such as Capi- 

 tella, a number of the dissepiments may at the breeding season 

 completely degenerate so that the cavities of the various 

 metameres concerned become perfectly continuous. The 

 ccelom of each metamere consists in reality of two sacs which 

 are folded around the digestive tract, which they enclose, 

 and come into contact with each other above and below the 

 intestine, forming the dorsal and ventral mesenteries (Fig. 99, 

 dm and urn). That wall of each sac which lines the muscula- 

 ture of the body-wall is termed the somatic layer of the peri- 

 toneum, while that surrounding the digestive tract is the 

 splanchnic layer. 



The blood vascular system consists of a dorsal vessel 

 (Fig. 99, db) which runs along the mid-dorsal line of the diges- 

 tive tract and which is frequently contractile in portions of 

 its course, serving as a heart, and a ventral vessel (ub) lying 

 below the digestive tract, and being connected with the dor- 

 sal vessel by lateral trunks, arranged metamerically. From 

 these vessels branches are distributed to the various regions 

 of the body. The blood is frequently colored, usually red, 

 and contains colorless corpuscles, the coloring-matter being 

 dissolved in the plasma in which the corpuscles float. The 

 blood vascular system is completely closed throughout its 

 entire course, never opening into sinuses without definite 

 walls. In addition to the blood which circulates within this 

 definite system of tubes the ccelom also contains a corpuscu- 

 lated fluid, frequently colored and approaching blood very 

 closely in its characters. This hcemolympTi contains corpus- 

 cles, usually amoaboid in form, and may circulate through the 

 body from one metamere to another through openings in the 

 dissepiments. In a few forms, such as Capitella, it may fulfil 

 the functions of the blood, a true blood vascular system being 

 wanting, and in this case contains, in addition to the colorless 

 amoeboid corpuscles, others which are disk-shaped and pig- 

 meuted. It seems probable, however, that the absence of a 

 true blood vascular system is a purely secondary phenome- 

 non, and accordingly does not indicate a primitive condition. 



The mouth is situated on the ventral surface of the body, 



/ * 



at the junction of the head metamere with the first trunk 



