48 M\'RIAPODA 



short palp or feeler. The mouth parts may have the forms 

 known as chewing, biting, or suctorial (^Polyzonium) mouth 

 appendages. 



With the exception of the terminal segment, and in many 

 cases the first or the seventh, each segment bears one or two 

 pairs of limbs. These may be very long, as in Scutigera, or very 

 short, as in Polyxenus. They may l^e attached close to one 

 another near the ventral middle line of the body, or may 

 have their leases far apart fi*om each other, as in the Chilopoda. 

 The exoskeleton or external armour is composed of chitin 

 (Chilopoda) or of chitin with calcareous salts deposited in it 

 (Chilognatha). 



Their internal structure has a great likeness to that of 

 Insects. 



The general position of the internal organs may be seen from 

 Fig. 28, which shows a Lithohius dissected so as to exhibit the 

 digestive and nervous systems. 



The digestive canal, which is a straight tube, extends through- 

 out the whole length of the body, and terminates in the last 

 segment of the body. It may be divided into the following 

 parts : 



1. A narrow oesophagus, beginning with the mouth or buccal 



ca\'ity, and recei\'ing the contents of two or more 

 salivary glands ((f). 



2. A wide mesenteron or mid-gut (w) extending throughout 



almost the whole length of the body. 



3. A rectimi which at its jimction with the mid-gut receives 



the contents of two or four Malpighian tubes (g, h) which 



function as kidneys. Their function was for a long 



time unknown, but the discovery of crystals of uric 



acid in them placed the matter beyond doubt. 



The heart has the form of a long pulsating dorsal vessel 



which extends through the whole length of the animal It is 



diA-ided into a number of chambers, which are attached to the 



dorsal waU of the body, and are furnished with muscles of a 



wing-like shape, which are known as the alary muscles, and 



which govern its pulsations. The chambers are furnished with 



valves and arteries for the exit of the blood, and sUts known as 



ostia for the return of the blood to the heart. The blood enters 



the chambers of the heart from the body cavity through the 



