400 



CHARACTERS OF INVERTEBRATE ANIMALS 



form in the main to the type which 

 Cockroach (p. 348). The heart is 

 to the upper surface of the body 

 containing pericardial space, from 

 through numerous pairs of valvular 



A. 



INTESTINE 



Fig. 247. Structure of Peripatus 

 A, Digestive organs (from below). B, Nervous system and 



has been described for the 



a slender tube placed close 



and suspended in a blood- 



which blood passes into it 



apertures. The rest of the 



blood -system consists of 



larger and smaller spaces 



which, together with the 



heart and pericardial cavity, 



make up a circulatory ar- 



rangement of which the 



parts communicate with one 



another. 



This appears to be a 

 suitable place in which to 

 speak more fully of the 

 nature of the Arthropod 

 heart, which is essentially a 

 blood-tube within a blood- 

 space with which it com- 

 municates by paired aper- 

 tures. In a Vertebrate 

 (p. 40) or a Mollusc (p. 308} 

 the heart possesses one or 



m re Wricks, IlltO which 

 blood IS pOUred by 



pericardial 



surrounding it does not 



contain blood at all. Professor Ray Lankester explains the 

 arthropod condition by supposing that the heart was originally 

 a tube receiving blood by several pairs of lateral vessels which 

 later on dilated into auricles where they joined the central tube. 

 The fusion of these auricles into a large space round the heart 

 would give the state of things now existing. The pericardial 

 space of, say, Peripatus is therefore to be regarded, if the 

 theory be well founded, as equivalent to a big auricle surround- 

 ing the heart; the physiological problem solved in this case 

 being the evolution of an arrangement for storing blood about to 

 enter the heart. 



Peripatus further agrees with typical air-breathing Arthropods 



