36 ME. M. LAXJEIE ON THE 



GrENEEAL CoNSIDEEATIOlirS. 



In the following pages I only propose to consider a few points 

 in Araclmid morphology on wliicli it seems to me that my 

 observations have thrown some light. Many points — such as the 

 existence of a number of pre-oral segments in the embryo — I 

 have not dealt with, because it seems better to wait for further 

 observations rather than to try and generalize on a manifestly 

 insufficient basis. 



Post-oral Thoracic Appendages. 



Gaubert *, in his recent paper on the Arachnids, treats of the 

 limbs of the terrestrial forms at some length, but his conclusions 

 do not appear satisfactory to me. He considers the typical 

 walking-leg of the Arachnids to consist of six segments, the 

 articulations between which are capable of dorso-ventral motion. 

 Antero-posterior motion has been acquired in most forms, but 

 always by the formation of a secondary joint, which has arisen in 

 various parts of the leg in different forms. Thus, in Pedipalpi, 

 Phalangidse, and Spiders the fourth segment has been divided ; 

 in Scorpions the fifth, and in Galeodes the third. That secondary 

 jointing does take place in some forms is certain, but that all the 

 articulations capable of antero-posterior motion are due to it I 

 doubt. In the figure on p. 37 I have drawn a number of legs of 

 different forms, a glance at which will make my views clearer than 

 pages of description. The numbers alove each figure are those of 

 the segments of the limb as I interpret them, those in brackets 

 below are according to Gaubert. The articulation capable of 

 antero-posterior movement is marked with an asterisk. In a pri- 

 mitive limb, then, for a type of which I will take that of one of 

 the Eurypterids, we have seven segments, of which the first is 

 modified for mastication, and the articulations of which are 

 capable of movement in any direction. Appendage ii. seems, in 

 contradistinction to the rest, to have only six segments in all 

 forms. The following are the chief modifications which have 

 taken place in the various orders : — 



(a) Eurypterids. — Appendage ii. may have a tactile function, 

 as in Slimonia. Appendage vi. is always larger than the rest and 

 usually flattened to form the swimming-foot. In Stylomerus, v. 

 and vi. are enormously elongated. An epicoxite is present in 

 some of the limbs. 



* Ann. Sci. Nat. ser. 7, vol. xiii. 



