SECT, xii RELATION OF APUS TO CRUSTACEA 193 



comparable with the sensory cnclitcs on the ventral 

 edge of the trunk limbs of Apus ; in the middle of 

 this group of sensory processes is a small pair of 

 chelae. This whole limb seems to combine loco- 

 motory with protective functions. Its tip lies close 

 to the lateral gap between the cephalo-thoracic and 

 abdominal shields, so that no enemy could enter with- 

 out being immediately perceived and thrust out. This 

 first trunk limb seems to have preserved its dorsal 

 sensory cirrus, also no doubt as a guard against the 

 entrance of enemies which could not easily be ejected 

 if once lodged on the dorsal edges of the head limbs^ 

 i.e. between the limbs and the shield. 



The last pair of limbs of the cephalo-thorax is highly 

 modified as a flat cover or operculum for the abdominal 

 feet. Its form is essentially Phyllopodan. In Limulus, 

 the passage from the flat Phyllopodan limb to the 

 Crustacean leg is abrupt ; there are no transition forms 

 as in Apus. The first trunk limb is leg-like, the second 

 Phyllopodan. As the form of the latter is essentially 

 the same as that of the abdominal limbs, we reserve 

 our description of it till later. 



Before leaving these cephalo-thoracic limbs we wish 

 once more to call attention to their arrangement, 

 which is well explained by the manner of life of 

 the animal. The animal, i.e. its anterior end, seems 

 as if fixed up in the vault of a roof, the mouth 

 being in the centre and the limbs hanging down all 

 round. The most anterior and most posterior limbs 

 do not function as jaws, but the five limbs between 

 these two, i.e. the last four head limbs and the first 



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