iS 94 .] 



THE BRITISH NATURALIST. 



5 1 



At a higher level than Myriapods is the somewhat heterogenous class, 

 the Arachnida, comprising some four orders : typical representatives ot 

 their species are scorpions, spiders and mites. This class includes 

 animals of extraordinarily different forms, from the scorpion to the minute 

 Tardigrada. Many of them, particularly amongst the mites, are parasitic, 

 whilst the great majority ot the species live upon the juices of other 

 animals. 



The number of the body segments throughout this class is considerably 

 reduced, and in all the species the tendency to coalesce is obvious. Their 

 total number never exceeds nineteen, and is usually fewer. It must 

 be understood that when we say that the class Arachnida follows closely 

 upon the Myriapods, it is not meant to assert that a Myriapod was the 

 immediate ancestor of the varied forms which are now grouped as 

 Arachnidans ; but that various modifications arose from various species 

 of Myriapods, and such of these as possessed four pairs of ambulatory 

 legs are called Arachnidans. It is an interesting fact that none of these 

 eight-legged descendants of Myriapod ancestors possess antenna?. Why 

 this should be it is impossible to say, but that it is, somehow or other, due 

 to the internal constitution of the animal is evident. All the other air- 

 breathing Arthropods, whether higher or lower, possess antennae, but 

 those which have eight legs never possess those organs of touch. 

 All the Arachnida possess chelicera?, which are homologous to the 

 mandibles of Myriapods and insects, and pedipalpi, which are homologous 

 to the maxilla? of those classes ; and since the pedipalpi though for- 

 midable weapons are frequently used as organs of touch, thus render- 

 ing the antenna? unnecessary, these latter have been first disused, and 

 have subsequently disappeared. 



As a justification for the assertion that all Arachnidans are descended 

 from ancestors which, if we could see, we should without hesitation 

 describe as Myriapods, we may refer to the fact, that embryo spiders 

 and scorpions have limbs on the abdominal somites. These legs, being 

 functionless in the embryo, and absent in the adult, surely point to a 

 many-legged ancestor. Since not a few of the smaller spiders float 

 through the air by means of delicate threads, which they exude, and trust 

 to the kind action of the wind to find resting-places, it is evident that 

 if the power of flight had been obtainable within the Arachnidan group 

 it would have been of great advantage to its possessors ; but seeing that 

 this group is essentially terrestrial, and that its progenitors have never, 

 since they emerged from the worm-type, resumed aquatic life, there has 

 been no chance of developing organs homologous to insects' wings, which 

 are nothing more than the modified gill-leaflets of a once aquatic form. 

 The tendency for adjacent trachea? to coalesce and form a sort of purse- 

 like pulmonary sac, incipient in some centipedes, reaches its culmination 

 in spiders and scorpions ; in the latter, trachea?, as such, have disappeared, 



