400 



CHARACTERS OF INVERTEBRATE ANIMALS 



SLIME DUCT- 



form in the main to the type which has been described for the 

 Cockroach (p. 348). The heart is a slender tube placed close 

 to the upper surface of the body and suspended in a blood- 

 containing pericardial space, from which blood passes into it 

 through numerous pairs of valvular 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 

 more auricles, into which 

 blood is poured by veins, 

 and the pericardial space 

 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 



Fig. 247. — Structure of Peripatus 



A, Digestive organs (from below), b, Nervous system and 

 excretory organs (from above). I. A. Position of intestinal aperture 

 (on under side). 



