226 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



chamber, and the gills project freely into the water, as 

 in the lower Crustacea ; they are attached to the five 

 pairs of abdominal feet, and are broad, flattened, and 

 provided with a number of secondary plates. These 

 gills are essentially formed by a hard supporting axis, 

 on which are placed some hundred and fifty flattened 

 lamellae (Fig. 97) ; these lamellae are exposed to the 

 surrounding water, and as they are hollow, and their 

 cavities contain blood, we have only further to know 

 that their walls are delicate to understand how it is 

 that these "gill-books" are respiratory organs. 

 Connected with these gills are the so-called " lung- 

 books " of the scorpion, which are adapted for aerial 

 respiration, and the exact characters of which have 

 been very carefully and ingeniously elucidated by 

 Lankester. In that Arachnid the ninth to the twelfth 

 segments bear appendages which are respiratory in 

 function, the appendages of the eighth segment, or the 

 first which is branchial in Limulus, being more espe- 

 cially modified as the so-called " pectines." These 

 respiratory appendages, or " lung-books," are, like the 

 " gill-books " of the king-crab, formed essentially by 

 a hard supporting axis, and a number of lamella set 

 on that axis (Fig. 97, B ; and in greater detail c) ; but 

 they differ, at first sight, altogether from those of the 

 king-crab in being no longer exposed freely, but 

 placed in recesses, which open to the exterior by a 

 narrow slit. This slit gives entrance not to blood but 

 to air, and, as it communicates with the cavities in the 

 lamellse of the lung-book, we expect to find that these 

 do not, as in Limulus, any longer serve as blood 

 passages ; the blood, indeed, is now found in the sac 

 which invests the lung-book. Great as are the struc- 

 tural changes, the ultimate physiological arrangement 

 is the same as before ; in other words, the lung-books 

 no less than the gill-books are respiratory organs, but, 

 instead of carrying the blood to the oxygen, they carry 



