1881.] 



On Stigmata in the King Crab. 



397 



this note, as to the practical identity of the gill-books of Limulus and 

 the lung-books of Scorpio, implicitly contain the affirmation that 

 either the structures of Limulus have been derived from those of 

 Scorpio, or those of Scorpio from those of Limulus, or that a third 

 (possibly now extinct) form has given rise to both Limulus and 

 Scorpio. Further, it is to be observed that such extinct form might 

 be more like to Limulus than to Scorpio, or vice versa, in respect of 

 any particular element of structure. 



To make a long story as short as possible, I may say that without 

 prejudicing the recognition of the (as I think) well-established mor- 

 phological identities above pointed out, we may best explain their 

 existence by assuming that an aquatic form breathing the dissolved 

 oxygen of the water inhabited by it, by means of book-like gills, was 

 the common ancestor of Limulus and of Scorpio. From the book- 

 like gills of this ancestral form, the broad series of Limulus and the 

 narrower lung-books as well as the pectinse or combs of the Scorpion, 

 have been derived. The form of the book-like gills of this Arachnidan 

 ancestor was probably something intermediate between the three 

 existent modifications of it — and best conceived of, perhaps, by ima- 

 gining the teeth of the Scorpion's " pectinate organs " to become soft 

 and flattened and increased in number. 



To obtain from these the Limulus gill, we have but to suppose 

 certain definite changes of dimension, the imbrication and character of 

 the lamellas and their external position remaining unaltered. 



To arrive at the book-lungs of the Scorpion, we have to imagine 

 the ventral surface on each side in close proximity to the short ap- 

 pendages carrying the gill-books — to have become deeply cupped ot 

 depressed ; so that two series of cup-like pits should be formed, a 

 right and a left, a pair being placed in each segment, corresponding 

 to each pair of gill-books. Each cup must have become so large in 

 area and so deep as to embrace within its limits the relatively small 

 adjacent gill-book. Further, when once the gill-book had been in- 

 volved in this cup-like depression, the walls of the cup must have 

 tended to grow together so as to form a pulmonary chamber with only 

 a narrow slit-like opening to the exterior, and pari passu with this 

 closing in of the cupped area — and the protection of the respiratory 

 lamellse — the Arachnid must have acquired the power of leaving the 

 water and of breathing the atmospheric oxygen admitted to the damp 

 chamber formed by the cave-like areas of depression. 



Whilst framing such a hypothetical account of the way in which 

 the transition from naked " gill-book " to in-sunken " lung-book " 

 could have taken place, one naturally asks — " Is it not a somewhat 

 gratuitous assumption to imagine that cupped areee should form con- 

 veniently by the side of the gill-books of the aquatic ancestor, so as 

 to be ready to increase in size and ultimately draw into themselves, 



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