VI ARTHROPODAN APPENDAGES 223 



interesting to note that in the young embryo they duly make their appear- 

 ance as small rudiments although they fail to proceed with their develop- 

 ment. The eighth appendages form a genital operculum (Fig. 99, B, VIII) 

 as in Limulus, only greatly reduced in size. The ninth appendages are 

 curious comb-like organs (pectines — Fig. 99, B, IX) the function of which 

 is unknown. Appendages X-XIII appear to be completely absent in 

 the adultj and in about the position where each of them should be there 

 is present a narrow somewhat obliquely-placed slit (stigma — Fig. 99, B, 

 X-XIII) which leads into one of the breathing organs or lungs. Each 

 lung is a small chamber into the cavity of which there project backwards 

 from its anterior wall an arrangement of thin leaves (lung-book) like 

 those of the gill-book of Limulus (Fig. loi, B). A fascinating light is 

 thrown on the evolutionary history of these lungs by the study of their 

 mode of development in the young Scorpion embryo, for it is found 

 that in place of each lung there exists for a time a definite limb rudiment, 

 which develops projecting plates on its posterior surface, agreeing exactly 

 with the rudiments of the gill-book in the young Limulus . In the Scorpion 

 however as development goes on the limb rudiment with its gill-book 

 ceases to project and becomes flush with the neighbouring surface, while 

 the leaves project into a depression of the surface, which gradually 

 deepens to form the lung cavity. 



Appendages of Crustacea 



To illustrate the series of appendages of the Crustacea we may take 

 those of the Fresh-water Crayfish {Astacus)?- The appendage is seen in 

 its least modified form about the middle of the abdomen, say the third 

 or fourth of the six abdominal segments (Fig. 102, B). Shaped hke an 

 inverted Y, it consists of a basal portion the protopodite, which bears at 

 its end two diverging branches, an outer, the exopodite, and an inner, the 

 endopodlte, each tapering to a point and divided up into numerous 

 segments. At each end of the abdomen the appendages are modified : 

 at the hinder end the last of the series, while showing the same three 

 parts as the typical appendage, has these parts broadened out into flat 

 plates which when spread out in the same plane as the flat plate-like 

 telson (Fig. 102, A, t) form with it an expanded kind of fin, the possession 

 of which enables the animal to shoot rapidly backwards in the water by 

 violently bending its abdomen in a ventral direction. At the front end 



1 Failing specimens of this animal the common Lobster (Homarus) or the 

 Norway Lobster [Nephrops) may be studied : they agree with Astacus in all 

 their main features. 



