" WIXD-SCORPIONSr . 333 



in the different Arachnids, fill up every available corner of 

 the abdomen left free by the circulatory, respiratory and 

 genital systems. Into this richly branched alimentary 

 system the liquid food is pumped to its utmost capacity, and, 

 owing to the diaphragm, without any danger of disturbing 

 the nervous and locomotory systems of the anterior region. 

 Further, there is some anatomatical evidence to show that 

 when the abdomen is distended with liquid food the ali- 

 mentary canal can be constricted as it passes through the 

 waist or diaphragm, as an extra safeguard to prevent the 

 liquid food from flowing back into that portion of the ali- 

 mentary system which runs through the anterior region. 



The habit of distending the abdomen to monstrous pro- 

 portions with liquid food seems common to all the larger 

 Arachnids. Individual specimens with distended abdomens 

 may of course be gravid females, but as often as not, they are 

 animals which had been killed just after a full meal. The 

 most familiar Arachnid thus distending itself does not, 

 however, happen to be one of the large forms, but a Mite. 

 I refer to the Ticks which, before fastening on to their 

 hosts, are quite insignificant in size, but, when once fixed, 

 suck in so much blood that they swell up to the size of a 

 bean. It seems, indeed, as if their tough skins were as much 

 an adaptation to prevent them from bursting through over- 

 feeding, as to defy the efforts of their hosts to scratch or 

 rub them off. 



The Degeneration of the Abdominal Limbs.— T\\^ 

 differentiation of the body into an anterior locomotory 

 and a posterior vegetative region sharply divided from one 

 another, has led to the degeneration of those limbs which 

 originally belonged to the segments of the posterior region. 

 We have every reason, however, to believe that, in the 

 ancestral form, they long persisted in a more or less useless 

 condition. A few of these aborting limbs have been utilised 

 by the different existing Arachnids, but in very different 

 ways. Those of the first abdominal segment generally 

 persist as protections for the genital aperture, as a rule 

 simplified to a pair of covering scales, but in Harvestmen 

 and certain specimens of Phrynus they form distinct 



