78 PART I. GENERAL ACCOUNT 



sometimes give the impression of large-grained coagulations (Fig. 49). Free 

 lymphocytes may often be encountered in the blood — rather small round or 

 oval cells with obvious nuclei. The lymphocytes often settle on the walls of 

 the vessels and sometimes they may be seen here in such numbers that one 

 may mistake them for elements of an endothelium. 



Circulation of the blood 



Although direct observation on the circulation of the blood is lacking, a 

 series of morphological features of the blood-vascular system make it possible 

 to arrive at reasonable conclusions about the direction of flow of the blood. 



As will later become apparent there is no alimentary canal in pogonophores, 

 its function being taken over by the tentacles. These organs also carry out the 

 respiratory function. In consequence the blood running out of the tentacles 

 must be more saturated both with foodstuffs and with oxygen than is the 

 blood which flows into them. The organs most in continuous need of food 

 substances and oxygen — the brain, and the ovary in the female — are supplied 

 by branches from the dorsal vessel, so that it is reasonable to suppose that 

 the blood in this vessel comes direct from the tentacles. Then the heart, 

 developed on the ventral vessel at the base of the tentacular crown, is the only 

 possible propulsive organ, pumping the blood through the whole system of 

 tentacular vessels. Clearly a muscular heart is necessary to overcome the very 

 great resistance of the fine capilliaries of the pinnules of the tentacular 

 apparatus (p. 80), a resistance which could not be overcome by the unaided 

 pulsation of the ventral vessel. Thus we may reasonably conjecture that the 

 blood moves forward in the ventral vessel, through the heart into the tentacles 

 and thence backwards from the head in the dorsal vessel (Ivanov, 1955a, 

 1960a). 



As we have seen (p. 63) contraction of the dorso-ventral musculature of 

 the protosoma probably in some measure assists, if not the circulation then 

 at least the renewal of the blood in the cephalic vessels. 



[The beating of the heart has been observed in living Siboglinum. Ivanov's 

 conjectures are confirmed, for the blood may be seen to pass from the heart 

 into the tentacles and thence back into the dorsal vessel — D.B.C.] 



