194 THE APODID^ part ii 



trunk limb, have powerful jaw pieces, which arc 

 arranged in two rows, one on each side of the oral 

 aperture, the dorsal parapodia being developed into 

 an outer ring of chelate feet for seizing prey and 

 bringing it between the jaws. 



It is clear that the efficiency of these long limbs, 

 already limited as to space for free movement, would 

 be materially lessened if on their dorsal edges they 

 had to carry gills, &c. ; hence these appendages have 

 entirely disappeared, respiration becoming localised on 

 the abdominal limbs, which have been especially modi- 

 fied for the purpose. In the Trilobites the movement 

 of the limbs is also limited by a large shield running 

 the whole length of the body, but in their case both 

 the gills and the cirri are retained, the reason being 

 very simple, viz., that the ambulatory legs of the 

 Trilobites do not require to carry out the complicated 

 movements of the limbs arranged round the mouth of 

 Limulus, but are simply ambulatory legs. 



Two processes from the ventral surface of the 

 body bring these limbs to a close. The mouth parts, 

 i.e. the masticatory ridges, work between the labrum 

 at the one end and these two processes at the 

 other. Although authors have recognised that these 

 processes function as an under lip, they hesitated to 

 assert that morphologically they were the true under 

 lip. We can, however, hardly doubt that they are to 

 be homologised with the under lip of Apus. An 

 examination of the special modifications of Limulus 

 explains the position of the organ ; the mouth has been 

 lengthened out longitudinally so that the labrum has 



