7. THE BLOOD-VASCULAR SYSTEM 73 



half of the trunk, beginning a little in front of the first girdle, the lateral 

 branches communicate with the ventral vessel. 



Throughout its length the ventral vessel keeps nearly the same diameter 

 and only at the very front end, in the protosoma, is it greatly distended to 

 form the muscular heart. In front of the heart it becomes slender, turns 

 towards the dorsal side and enters the protocoele. Here it forks into two 

 symmetrical branches — the vessels of the tentacular crown, from which start 

 the afferent vessels of the tentacles (Pis. II, III). 



Two vessels — afferent and efferent — run in the coelomic canals of each 

 tentacle, communicating with each other by means of the capillaries of the 

 pinnules (p. 80, Figs. 53-55). The efferent vessels from the tentacles open 

 into the pair of efferent vessels in the protocoele, which in turn pour the 

 blood into the dorsal blood vessel (Pis. II, III). 



The walls of the blood vessels consist of a structureless membrane, which, 

 as a rule, is connected with the system of membranes bounding the peri- 

 toneum or forming the basement membranes of the epidermis. This intima 

 is noticeably thicker in the dorsal and ventral vessels. There is no endo- 

 thelium (Figs. 22, 45, 49). Large often isolated cells may, it is true, frequently 

 be encountered on the internal surface of the vessels, but these, however, 

 appear to be temporarily attached cellular elements of the blood. Both main 

 vessels are clad in a layer of circular muscle which has an unusual structure. 

 Its myocytes are of the nematode type, with a cytoplasmic part, containing 

 the nucleus, projecting into the body cavity, and a comparatively narrow 

 spindle-shaped contractile fibre possessing true transverse striations. Such 

 muscle fibres are to be found all along both the longitudinal vessels (Figs. 

 44, 46). The striated musculature is particularly well developed on the ventral 

 vessel and in particular on its posterior half, where the large cytoplasmic 

 perikarya of the myocytes cover the vessel in a complete layer (Figs. 22, 

 46, 50). 



The heart 



In appearance the heart is simply a thickened portion of the ventral blood 

 vessel, with a particularly powerfully developed mantle of muscle fibres. Its 

 size is directly correlated with the degree of development of the tentacular 

 apparatus. Where most tentacles enter into the structure of the crown the 

 heart is biggest. In the unitentaculate Siboglinum it is poorly developed 

 (PI. I); in Polybrachia, Lamellisabella (PI. Ill) and Spirobrachia it reaches a 

 considerable size. 



The lumen of the heart is bounded by a comparatively stout membrane 



