COMPARATIVE MORPHOLOGY OV TILE GALEODID-E. 401 



completely vanishes ; i. e., in other words, the resemblance between the specialized forms . 

 is purely convergent. 



Relation of Modern Arachnids to the Hypothetical Ancestral Form. — Having thus 

 sketched a form which embodies aU the more important specializations common to all the 

 leading types of Araclmids, at what appear to be their least specialized stages, we can 

 now comjmre the existing Arachnids with this form as a measure of their respective 

 specializations. The task is not easy, inasmuch as every form retains some primitive 

 features, while others are specialized, and it is not always possible to appreciate the 

 morphological worth of the different characters so as to be able to say which precedes 

 the other. 



We may at once dismiss the Phalangidae and the Acaridae as undoubtedly the most 

 specialized Arachnids, and confine our comparison to the Pedipalpi (esp. Thelyphonus), 

 the Galeodidae, the Scorpionidie, the Araneae, and the Chernetidse. Each of these 

 retains important characters which have some claim to be primitive. 



Thelyphonus and Schizonotus. — This form has retained perhaps the largest number of 

 the most important primitive characteristics. Schizonotus especially retains the cephalic 

 lobes, two (iierhaps three) free cejDhalothoracic segments, and the leg-like position of the 

 pedipalps, which, moreover, have only very rudimentary chelae. Further, only nine 

 segments of the abdomen are swelled into a distensible sac, with traces of limbs or 

 stigmata on eight of them. Internally, Thelyphonus retains the largest number of 

 ostia in the heart and the largest number of alimentary diverticula. On the other 

 hand, the fusion of the bases of the pedipalps so as to enclose the beak, the enormous 

 backward development of these bases, squeezing up the sternite of the 3rd segment 

 with the coxae of its limbs, so that they no longer show on the ventral surface, are 

 highly specialized characters. In addition, we have to add to this specialization the 

 sensory character of the 3rd pair of appendages, the highly-developed waist, and the 

 caudal filament, which, however, may perhaps be a development of an anal cirrus of the 

 ancestral form. 



Galeodidae. — These are primitive in the retention of the cephalic lobes, of three free 

 cephalotlioracic segments, and of variously-sized portions of the tergum of the 3rd segment,, 

 peeping out from under the cej)halic lobes ; further, in the simple form of waist, hardly 

 disturbing the serial arrangement of the segments ; in the retention of leg-like pedipalps, 

 of cephalotlioracic tracheae, and of traces of limbs on all the abdominal segments. 

 Internally, the conditions of the endosternite, a long heart, with two (traces of three) 

 ostia m the cephalothorax, the paired and separate genital glands are primitive 

 features. 



On the other hand, the Galeodidae are highly specialized in the great size of the 

 chelicerae, in the approximation of the coxse of the limbs, and consequent obliteration of 

 the sternites along the whole length of the body, in the swelling of all the abdominal 

 segments to form a distensible sac. 



Scorpions. — The Scorpions have retained the primitive number of segments, with 

 distinct filamentous remains of the legs of the 2nd abdominal segment (pectines), and 

 ? of the 12th segment (sting), while only seven abdominal segments are distended into- 



SECOND SERIES. — Z00L0G1% VOL. VI. 53 



