102 NATURAL SCIENCE. February. 



looked upon as a third group of arthropods, derived from a special 

 branch of the chief stem. 



Similarly the Arachnoidea must hold the position of a fourth 

 arthropod group. As is well known, different views have been taken 

 as to the relations of the Arachnoidea. According to those authors 

 who make a fundamental difference between gill- and tracheal respira- 

 tion, they form, in connection with the insects, myriopods, and 

 Onychophora, a sub-phylum of the Arthropoda, viz., Tracheata. The 

 second sub-phylum is that of the Branchiata, which includes, besides 

 the Crustacea, the Gigantostraca, Trilobita, and Xiphosura (Limulus). 

 On the other hand, a second school of authors holds the Arachnoidea 

 to be related to the three last-named groups, and classifies them 

 together under the head Palaeostraca. This conception, which I hold 

 to be the right one, depends on the remarkable agreement shown by 

 the segmentation of the body and morphological structure of the 

 Palaeostraca and the number and position of their limbs with what 

 obtains in the scorpions; and it is furthermore strengthened by the 

 similarities observed between the internal structure of Liimdus and 

 Scorpio (endoskeleton, coxal glands, structure and position of gill- 

 and lung-books). The highly organised Arachnoidea would therefore 

 stand nearest to the oldest and most original forms of this group of 

 arthropods, and have developed from water-living Branchiata into 

 terrestrial Tracheata. As regards the transformation of the gill-plates 

 carried in chambers on the ventral surface into the lamellae of the 

 so-called lungs or tracheal chambers, it seems to me that McLeod has 

 given a likely explanation. 



But now the question comes, — are we to consider this fourth 

 arthropod group as an independent branch of the arthropod stem, or 

 are we to derive it from the Protostraca ? The latter view would 

 commend itself if we could succeed in discovering a nauplius-like 

 larval form in the young stages of the Palaeostraca. Of course the 

 early stages of Limulus have been compared to trilobites, and on the 

 other hand the oldest and most primitive trilobite {Protaspis), the 

 larval nature of which cannot well be doubted, has been described as 

 the forerunner of the crustacean nauplius and called Protonauplius 

 (Beecher). Nevertheless, satisfactory proofs of this very interesting 

 point are wanting, especially the presence of limbs, the structure and 

 position of which are not known. So long as the necessary proofs 

 are in abeyance, we must attach greater probability to the first of our 

 suppositions, and thereby look upon the Arachnoidea and their pre- 

 decessors the Palaeostraca as a group arisen independently and 

 having no connection with the Crustacea. 



It is not necessary for me to emphasize the fact that in my 



discussion I have not dealt with proved facts, but only with views 



which claim a right to be heard, in deciding the relationship of the 



classes of Arthropoda from a standpoint based on the theory of descent, 



Vienna. C. Claus. 



