78 PHYSIOLOGY OF THE INVERTEBRATA. 



and two pairs of mofe delicate jaws — the maxillae. These 

 are the side lobes of the labium of the centipede, specialised 

 into distinct lateral jaws. In the Arachnida, the mandibles 

 are extended into prehensile and offensive claws. The 

 maxillye in the spider are related in a remarkable manner 

 to the function of reproduction. The specialisation is there- 

 fore incomplete. The Crustacea have a labrum, two mandibles, 

 four maxillae (the second pair representative of the split 

 labium of the Myriapuihi), and three pairs of maxillipedes 

 (feet-jaws). These last represent the six legs of the Insccta. 

 The somites, which in the latter bear motor organs, carry 

 in the Crustacea organs that serve for mastication, but they 

 are in structure closely allied to the true legs on the suc- 

 ceeding somites." 



In the Insecta, the labium, labrum, mandibles and maxilla? 

 are all met with ; they present numberless and complex 

 modifications, but for all that are chiefly subservient to the 

 functions of taking in or crushing food. (5) The odontophore 

 or radula of tlie Mollusca forms a still further advance, as it 

 appears to combine the functions of teeth and of a tongue. 



As far as a stomach is concerned, the first indication of it 

 as a separate organ is observable in some of the Echinodermata, 

 e.rj.^ in the Astcridea, but the specialisation of the organ is in- 

 complete, inasmuch as it forms a dual function, viz., that of a 

 renal organ as well as being a gastric cavity. There is a true 

 stomach (a dilatation of the alimentary canal) in the Annelida, 

 Arthropoda, Puhjzoa, Brachiojwda, and Mullusca. The crop 

 present in the Insecta is simply a cavity which serves to store 

 the food before passing into the stomach. The intestine is 

 straight, and without convolutions in many forms — as, for 

 example, in the Astcridea, Myriapoda, Arthroejastra, &c. — but 

 also in many of the Mulhisca there is a bend or flexure in the 

 intestine. 



