396 Prof. E. R. Lankester. [June 16, 



homologous with the post-anal sting-spine of Scorpio, or the jointed 

 post-anal filament of Thelyphonus, and if it may be compared with 

 anything in Astacus it is with the post-anal joint of that Crustacean's 

 telson. 



The true "telson" of Limulus embraces the larger part of the 

 unsegmented area, the area of potential segmentation which extends 

 from the sixth pair of movable lateral spines and the axial area from 

 the base of the last pair of gill-plates backwards. In the embryo 

 this area has one or possibly two segments marked off from it, so 

 that the small area formed by their re-fusion in the adult is, strictly 

 speaking, not " telson." 



The whole pree-anal region of the telson of Limulus (together with 

 the two embryonic segments of which all trace is lost in the adult) 

 corresponds to the six proximal (proximal that is to the anus) prae- 

 anal body-rings of Scorpio, of which that including the anus should 

 be called " telson." That this bulky region should remain unseg- 

 mented in Limulus, whilst giving rise to several segments (some 

 elongate and annular) in Scorpio, is no difficulty in the way of the 

 assimilation of the two forms, so far as Arachnidan characters are 

 concerned, firstly, because in the Spiders, which are typical Arachnida, 

 the abdomen is obscurely segmented in a manner parallel to that 

 observed in Limulus ; and, secondly, because in the Eurypterina (the 

 close affinity of which to Limulus we are permitted to assume as 

 demonstrated) the exact number of segments and an approximation 

 to the form, shown by Scorpio in this part of the body, is found. 



Whilst the term telson had, it seems, best be reserved for a larger 

 area than that occupied by the post-anal spine, it is to be noted that 

 Professor Owen has urged, not by the mere implication of a name, 

 but explicitly, that the post-anal spine of Limulus corresponds to the 

 several pras-anal segments of the Scorpion's tail as well as to its 

 " sting," and he has pointed to indications of annulation in the struc- 

 ture of this spine. The traces of annulation appear to me to be pre- 

 cisely similar in character to the angulations of the jointed post-anal 

 filament of Thelyphonus, and it appears to me that there is the same 

 inducement when the comparison of general form only is made in the 

 case of Thelyphonus compared with Scorpio, to regard the narrow- 

 jointed post-anal " tail " of the former as the equivalent of the latter's 

 tail, consisting of true prse-anal body-segments, as there is when 

 Limulus and Scorpio are compared, to assimilate the king crab's 

 spine to the segments of Scorpio just named. In yielding to the 

 inducement in either case, we should, I think, mistake a resemblance 

 of superficial adaptation of form for one of true genetic equivalence. 



II. — The Para-branchial Stigmata op Limulus. 

 The views which have been advanced in the preceding section of 



