PHYSIOLOGY OF THE INVERTEBRATA. 191 



In many of the Polychceta, the pseudo-haemal system is 

 entirely absent (e.g., Polyrw'e squamatd), while in others it 

 varies greatly in the arrangement of the principal vessels ; 

 " but they commonly consist of one or two principal longitu- 

 dinal dorsal and ventral vessels, which are connected in each 

 somite by transverse branches. Where branchiae exist, 

 loops or processes of one or other of the great trunks enter 

 them." The dorsal and ventral vessels are generally con- 

 tractile ; and the direction of the contractions " is usually such 

 that the blood is propelled from behind forwards in the dorsal 

 vessel, and in the opposite direction in the ventral vessel ; but 

 the course which it pursues in the lateral trunks is probably 

 very irregular." 



The Aethropoda. 



The various classes belonging to the Arthropoda present a 

 system of vessels, partially at least, shut off from the somatic 

 or body cavity. But the blood-vascular system is not com- 

 plete in any Invertebrate animal. In some part or parts of 

 the body the vessels will be found to terminate, and the blood 

 will flow through lacunae or spaces not bounded by any 

 limiting membrane. From this remark it will be observed 

 that the old form of circulation once more comes uppermost — • 

 i.e., the blood passes into the general body cavity. This 

 primitive form of circulation is met with in all Invertebrates, 

 but the higher forms have partially developed a system of 

 blood-vessels, which is, however, incomplete, consequently the 

 lower the animal, the more extensive is the lacunar circulation. 



In the InvertebrcUa, the arteries have not the three coats, 

 such as are met with in the higher animals. The heart is 

 generally situated in a dorsal position ; and its pulsations drive 

 the blood at once over the body generally, and not to the 

 organs of respiration first. " The word ' pericardium,' used 

 by some writers in describing the blood systems of the Inver- 

 tebrata, is an unfortunate and a misleading one. The 

 pericardium of the Insecta and Crustacea has no homology 



