300 Herr J. Wagner on the 



development of the nervous system and especially of the 

 eyes, the development of the alimentary canal, the deve- 

 lopment and the structure of the coxal glands, and so on*. 

 Everything convinces us of the close relationship between 

 Limulus and the Arachnids ; yet, just as Peripatus is no 

 Millipede, so also Limulus is no Spider, although Ray 

 Lankester also, by reason of the similarity between it and 

 the Scorpion, attempted to assign it to the Arachnids (No. 55). 



On the other hand, palseontological investigations have 

 proved beyond doubt that certain of the paleozoic Xipho- 

 sura (the Hemiaspidaj), which (as, for instance, Prestwichia) , 

 as is shown by the embryology of Limulus^ were closely 

 allied to the forms from which the Limulidse are derived, on 

 the one side resembled certain Trilobites, while on the other, 

 owing to the relatively smaller size of the cephalic shield, to 

 the complete segmentation of the pre-abdomen and abdomen, 

 as well as to their caudal spine and probably also the general 

 character of the appendages, they were nearer akin to the 

 Gigantostraca. If forms such as Prestwichia constitute the 

 transition in question, on the other hand, as we are well 

 aware, the Scorpion, on account of the general configuration 

 of its body, the proportionate size of its divisions, and the 

 character of its appendages, shows much closer affinity to the 

 Gigantostraca than to the Xiphosura ; wherefore it must be 

 assumed that also in their internal structure the Gigantostraca 

 displayed still more similarity to the Scorpion than did the 

 Xiphosura. 



Among the scientists by whom the view of the common 

 origin of the existing Tracheates is maintained, von Kennel 

 and Lang are the best known and most influential. The 

 authors in question have defined their views and stated them 

 in detail, the former in a paper (No. 24), the latter in his 

 ' Lehrbuch der vergleicheuden Anatomic ' (No. 38) ; both 

 writers, while differing from each other in their ideas as to 

 the relation between the Branchiates and the Tracheates, 

 agree generally in their views as to the origin of the Arachnids. 

 According to von Kennel's supposition, besides other branches, 

 those formed by the Annelids and the Crustaceans (Bran- 

 chiata) arose from the hypothetical Protrochosphsera, and 

 both branches developed upon parallel lines, but inde- 

 pendently. From the various forms or families of Annelids 

 arose the Peripatus-Wke, ancestors of the Tracheata, which 

 divided at the very beginning into two branches : of these, 

 one was constituted by the forms of the tracheate Arthropods 



* For a precise resumS of the comparisons between Limidns and the 

 Arachnids, see Kingsley's paper (No. 28). 



