424 On the Development of the Pedrpalpi. 



adult Thelyplionus in ap]Dearance that it may vrell be teruied 

 a larva with some degree of justice. 



In colour it is yellowish white. The abdomen, which is 

 cylindrical in shape in consequence of the still abundant yolk 

 which is stored up within it, materially exceeds the cephalo- 

 thorax in volume. Upon the cephalothorax are situated 

 three pairs of pale red, tube-like, ambulatory limbs, which 

 exhibit no distinct segmentation, and at their tips instead of 

 the claws bear adhesive disks of considerable size. The pedi- 

 palpi, like the unsegmented caudal filament, are still rela- 

 tively short ; their terminal joint, which in the adult is 

 developed into a powerful chela, is still unpaired. The eyes 

 still lie beneath the larval skin, while the lung -sacs do not 

 yet communicate with the exterior. 



Particularly striking, however, is the primitive condition 

 of the ventral nerve-chain, which is distinctly perceptible 

 beneath the thin chitinous covering ; for while the adult 

 animal exhibits only a single large ganglion in the cephalo- 

 thorax, from which, besides a few lateral branches, a simple 

 central main cord runs through the abdomen, to expand 

 posteriorly into a small ganglion, the larva still possesses a 

 completely segmented ventral chain. The six pairs of ganglia 

 of the cephalothorax are still sharply distinguishable from 

 one another ; the abdominal section consists of ten pairs of 

 ganglia, of which the first six pairs are connected with one 

 another by transverse and longitudinal commissures, while the 

 four terminal ones, which are more closely united together, 

 constitute a common but still segmented mass. 



It is only during this larval period that the development 

 of the organs is completed in the form in which we find 

 tliem in the adult Iheltjphonus. While this is taking place 

 the larva remains constantly upon the body of tiie mother, 

 and in the meantime consumes the yolk-material which it has 

 brought with it. After some time a second ecdysis takes 

 place, whereupon the creature, now equipped with all tlie 

 attributes of the parent, leaves its mother, hencefortli to seek 

 its ibod independently in the same pugnacious manner. 



It will be seen from what has been stated above that the 

 mode of development of Thelyplionus exhibits a greater 

 agreement with tliat of the true spiders than with that of the 

 scorpions. The pronounced divergence of the two lialves of 

 tiie primitive streak and the otlier phenomena connected 

 with tliis are characters which have to be taken into con- 

 sideration in this connexion equally witli the entire absence 

 of embryonic membranes. 



In the detailed ])apcr it will be my task to enter more fully 



