136 MALCOLM LAURIE. 



Chordata. The nerve-cord instead of peeling off from the 

 superficial layer of epiblast sinks down bodily, and is covered 

 by a layer of epiblast which grows over it from each side. 



The development of the coxal gland leaves, I think, no room 

 to doubt that it is a nephridium. That of the genital tubes is 

 less conclusive, but I should think it probable that they are 

 also, at least in part, nephridial. 



The gill-books are undoubtedly appendages comparable to 

 the abdominal appendages of Limulus. Whether they are 

 really invaginated, i.e. whether the edge of each lamella in the 

 Limulus appendage corresponds to the bottom of the fold 

 between the lamellae in the Scorpion's gill-book, or whether the 

 whole appendage has become sunk in a hollow in the abdominal 

 surface without being invaginated, it is difficult to say. Un- 

 doubtedly, the surface now exposed to the air has always been 

 the external surface, but that would be the case with either of 

 the above modes of derivation. Although the second alterna- 

 tive has the advantage that it is easy to see how the change 

 could take place gradually, I am inclined to think the first is 

 probably the true way in which they have arisen. One argu- 

 ment in its favour is that if the second alternative were correct 

 one would expect the gill-book to commence as a distinct out- 

 growth, which would become sunk in a pit. Now, there is no 

 such outgrowth in the formation of the gill-book. The first 

 thing to appear on the thickened portion of the epiblast, from 

 which the gill-book is formed, is a pit (PI. XVII, fig. 41). The 

 lamella? do not begin to form till a later stage. Again, the 

 abdominal appendages of Limulus are directed towards the tail 

 as one would expect abdominal appendages to be. Now, if the 

 appendage had sunk in without invagination, one would expect 

 it to be still directed towards the tail unless there were some 

 very good reason for its having changed its direction. If, on 

 the contrary, it had become invaginated it would naturally be 

 directed in the opposite direction towards the head, and this is 

 what we find in the Scorpion. The inpushing is from the 

 beginning towards the head, and the aperture opens towards 

 the tail (PL XVII, fig. 47). I think it is quite conceivable 



