THE EMBRYOLOGY OF A SCOEPION. 123 



a well-developed process extending inwards towards the middle 

 line. This is undoubtedly the sternocoxal process, which is 

 present on the second, third, and fourth appendages of the 

 adult. Lankester^ characterises the presence of this process 

 as a very important point of resemblance between the thoracic 

 appendages of Limulus and Scorpio. It is therefore interest- 

 ing to find it at this early stage present on all four pairs of 

 ambulatory appendages. A series of sections through the base 

 of the fifth appendage, i.e. third ambulatory (PL XVI, fig. 34, 

 a — h), shows the first stage of another structure characteristic 

 of Limulus and the Arachnids — the coxal gland. This consists 

 of a simple tube opening to the exterior at the base of the fifth 

 appendage (fig. 34 a), and running forwards through the meso- 

 blast to open in fig. 34 h into the coelomic space. There can be 

 no doubt that it is a nephridium. GuUand's researches" on 

 the coxal gland in the young Limulus point to the same con- 

 clusion. I have been unable to find traces of nephridia in 

 any other somites, unless, indeed, the genital tubes are partly 

 nephridial. The six abdominal segments also bear appendages 

 (Pi. XVI, figs. 32 and 35) . These appear on surface view much 

 more prominent than they really are owing to their white 

 colour, which is due to the greater thickness of cells. In 

 section (PL XVI, fig. 35) they are seen to project very slightly, 

 and to be formed by a thickening of the epiblast and somato- 

 pleure, but with no definite outpushing such as there is in the 

 thoracic appendages. The first pair of these appendages — the 

 genital opercula — is very small, and concealed by the last pair 

 of walking legs. The other five pairs — the pectines and four 

 pairs of lung-books — are all of approximately equal size and 

 structure. I have been unable to find the smallest trace of 

 appendages on the somites behind these, i.e. somites 13 — 17, 

 and do not believe they exist. 



The cephalic segment is not so deeply cleft as in the last 

 stage, and the mouth has shifted posteriorly so that now it lies 

 between the bases of the chelicerse. In the centre of each 



' ' Limulus au Arachnid,' p. 20. 



^ ' Quart. Jouru. Micr. Sci./ vol. xxv. 



