MORPHOLOGY OF ANIMALS 



arthropods this generally consists of a thick-walled, pul- 

 satile tube lying on the dorsal side of the body and extend- 

 ing through several somites; in each somite are a pair of 

 openings, the ostia, which open into the heart from the 

 pericardium, and through which returning blood enters 

 the heart (Fig. 13, B). In the moUusks also the heart is 

 of a compact type and is divided into auricular and ven- 

 tricular portions. Primitively two auricles are present, 

 though in some gasteropods this number is reduced to one ; 

 in all mollusks there is but one ventricle (Fig. 13, A). In 

 primitive arthropods and mollusks the blood flows out of 

 the ventricle at both its anterior and posterior ends; in 

 more highly differentiated members of these phyla, out of 

 the anterior end only. The vascular system of arthropods 

 is not a continuous system of vessels; the arteries soon end 

 in lacunar spaces in the tissues, and from these spaces the 

 blood is gathered into large sinuses and thence flows back 

 to the heart. These lacunar spaces and sinuses are not true 

 vessels, since they do not have definite walls, but they are 

 derived from the primary and secondary body-cavities; 

 the circulation is therefore an open one. In mollusks the 

 vascular system is more extensive than among arthropods, 

 but here also the circulation is open, the arteries being 

 connected with the veins by a system of lacunar spaces 

 instead of by capillaries. Finally among the echinoderms 

 and chordates the circulation is closed as among the anne- 

 lids; that is, the blood throughout its entire circuit is con- 

 tained within definite vessels. 



Circulation and Respiration. The manner in which 

 blood is supplied to the respiratory organs is of great im- 



