1881.] 



On Stigmata in the King Crab. 



395 



the cephalo-thoracic tergites. It has, however, a distinctly emarginated 

 tergal surface corresponding to it in the abdominal carapace. This 

 emarginated area is much more distinct in the embryonic condition 

 than in the adult. In Limulus, the median portion of the base of the 

 genital operculum and of the following five lamelliform organs 

 appears in each case to be a projecting ridge of the sternal surface 

 simply, whilst the distal and lateral area only (right and left of the 

 shallow median fissure) are to be regarded as representing a right and 

 left " appendage." The exact limit of the two elements, that is, of 

 the appendage proper and of its sternal base, can only be decided by 

 an exact knowledge of the development of the parts in the embryo. 



Abdominal Carapace and Telson. — In young embryos of Limulus 

 there are, according to Packard's observations, eight, if not nine, dis- 

 tinct segments indicated in the tergum of the abdominal region, 

 besides the rudiment of the post-anal spine. Of these the first repre- 

 sents the tergite of the genital operculum, and, in the adult, is in- 

 dicated by an emarginated area and immovable spine or angular pro- 

 cess on either side of the fused mass forming the abdominal carapace. 

 The next five embryonic segments correspond to the five pairs of 

 appendages carrying the "biblioid" gills, and are indicated each in 

 the adult by a movable spine, at the side of the tergum, and by five 

 areas on its free or pleural lower surface, marked out by the disposi- 

 tion of the hairs which clothe it. 



The next following embryonic segment (the seventh) is indicated 

 in the adult by a pair of lateral movable spines on the tergum (the 

 sixth pair of movable spines), but there is no further indication of 

 segments ; what was seventh, eighth, and ninth segments in the em- 

 bryo is fused into one piece, together with the segments in front, and 

 exhibits no rudiment of the former segmentation in the form of ridges 

 as do the first six abdominal segments. It forms an oblong area of 

 considerable bulk, not much smaller in its axial portion than the whole 

 of the abdominal segments in front of it. In the soft integument 

 beyond this is placed the anus and beyond that the anal spine. 



The word "telson" has been applied, in my opinion, unwisely to 

 the post-anal spine. The telson of such a Crustacean as Astacus is 

 that region of the body following upon the segmented region and 

 carrying within its area the anus. It is (as more especially pointed 

 out by Claus) a region which may be characterised as one of potential 

 segmentation ; it is not precisely a segment, but something more than 

 a segment, viz., that tract of the hindmost region of the body which 

 can, and in incompletely developed forms does, give rise to new 

 segments, by a process of separation from its anterior border. It has 

 necessarily, and by definition, a certain extension anteriorly, that is, in 

 front of the anus. 



Now, the post-anal spine of Limulus has no such character. It is 



VOL. XXXII. 2 E 



