VI ARTHROPODA 217 



actively on rocks and walls near the sea — the abdominal appendages are 

 reduced to small simple vestiges. In the embryo of the higher insects 

 the same vestiges may be seen^ but in the adult they have disappeared 

 completely so that the abdomen is limbless. 



In the Arachnida we find the same tendency, although some of the 

 abdominal appendages may persist in the Spiders in the form of little 

 finger-like organs for the manipulation of the threads of the web, or, as 

 will be shown later, in the form of special breathing organs. 



The persisting appendages show in each of the three main subdivisions 

 of the Arthropoda modifications in relation to differences in function 

 which afford extraordinarily interesting studies in morphology.^ We 

 will sketch these modifications in outline, keeping the three subdivisions 

 separate and so avoiding the comparison — which the present writer 

 believes to be fallacious — of the individual appendages, and still more 

 of parts of appendages, in one group with those in another. While 

 avoiding such comparisons in detail, it is important to bear in mind that 

 all three groups present the common feature already alluded to, that 

 the jaws used for masticating the food are modified appendages. 



Appendages of the Arachnida 



Within this group the series of appendages is seen in its least modified 

 form in Limulus (Fig. 98, B). The first pair of appendages, the chelicerae 

 (I), are small nippers placed just in front of. the mouth. The nipper at 

 the end of the appendage is formed by the penultimate segment being 

 prolonged into a kind of prong alongside the terminal segment. The 

 terminal segment can be pressed strongly against this prong so as to 

 take a tight grip of any object between them. A nipper constructed in 

 this fashion is known technically as a chela, and a limb possessing it is 

 described as chelate. The next five limbs on each side (Fig. 98, B, II-VI) 

 are large walking legs. In the female these are all chelate except 

 the last : in the male, however, II is without a chela and ends in a 

 stout swollen claw. VI is provided near its tip, in both sexes, with a 

 number of flat plates which the king-crab uses in burrowing into the 

 sand. 



These appendages (II-VI) are arranged round the sides of the mouth. 



1 The word morphology — ^invented (1807) by Goethe, poet and naturalist — 

 is used to designate the philosophical side of anatomy. Anatomy deals with 

 the " unmitigated facts " of structure : it becomes morphology as soon as the 

 attempt is made to correlate these facts of structure with underlying prin- 

 ciples — such as, above all, evolutionary history. 



