STRUCTURE AND FUNCTION IN INVERTEBRATES 



a system, unlike that of pelecypod moUusks, lacks veins; arteries extend from 

 the heart to all parts, and from these blood percolates through the tissues into 

 the hemocoel. Return to the heart involves certain tubular specializations of 

 the hemocoel, plus the pericardial sinus. In insects the system is even 

 simpler, because there are no arteries, although the anterior extension of the 

 heart may be termed an aorta. A unique feature of the arthropod system is 

 the manner in which the pericardial chamber has become a part of the 

 hemocoel; blood enters the heart from this cavity through small openings in 

 the heart wall termed ostia. These admit blood to the heart during the 

 diastolic phase of its beat, and the backflow of blood during systole is pre- 

 vented by the action of valve-like flaps covering the ostia. In pelecypod 

 mollusks the pericardial cavity is not involved in the circulatory function; 

 present in the ventricle wall are ostia through which blood enters from a 

 pair of auricles or atria which are a part of the venous system. In some 

 tracheate or air-breathing arthropods the circulation through the hemocoel 

 is aided by dorsal and ventral perforated membranes, termed diaphragms. 



Mouth 



Mouth 



Anus 



Fig. 17.3. Invertebrate circulatory mechanisms, continued. .4, the function of transport 

 served by fluid in the pseudocoel, circulated by movements of the body (Aschelminthes). B, 

 an "open" circulatory system in which a pulsatile ventricle and distributing; vessels con- 

 duct blood to the tissues, where it flows into sinuses; blood is returned to the ventricle throus;h 

 vein-like channels and a pair of auricles or atria (pelecypod mollusks). C, an open system 

 with a tubular heart bearing lateral ostia through which blood enters the heart; blood flows 

 anteriorlv into the intercellular spaces of the hemocoel, eventuallv returning to the heart 

 (Insecta). 



515 



