4o6 



ANIMAL RESPIRATION 



Phyllopods. The second jaw belonging to the head (first maxilla) 

 is provided with a large fan-like plate which by its movements 



helps to maintain a stream of water 

 through the space between the halves of 

 the shell {fig. 536). 



In Barnacles (Cirripedes) the long 

 tendril-like feet which fish for food (see 

 p. 254) probably assist in breathing, 

 which is also partly carried on by the 

 soft fold of skin that lines the shell- 

 plates on either side. In Acorn- Bar- 

 nacles (Balanidce) there is a folded pro- 

 jection between each of these flaps and the bases of the limbs, 

 which probably acts as a gill. 



Fig. 536. — A Mussel-Shrimp {CyJ>ris), 

 enlarged, ant.i, Antennule: ant.2, an- 

 tenna; MND., mandible; mx.i, first maxilla 

 (notice the large fan-plate) ; MX.2, second 



KING-CRABS (Xiphosura) 



The curious King- Crabs (Limulus) that shuffle about on. 

 the surface of mud, devouring worms, &c., are protected by 



strong shield -armour, through 

 which breathinsf cannot take 



place. Specialized gills are there- 

 fore present, and are well pro- 

 tected, as the habits of their 

 owners would otherwise render 

 them liable to be soiled with 

 mud. On examining the under 

 side of a King- Crab, a broad 

 plate, the gill-cover (operculum), 

 will be seen behind the last pair 

 of walking-legs (fig. 537). There 

 is good reason for thinking that 

 this plate has been formed by the 

 fusion of two flattened limbs. 

 Behind it may be observed the 

 projecting edges of the five pairs 

 of abdominal limbs, which also 

 are plate-like, and bear on their upper sides (so as to be well 

 protected) a large number of delicate gill-folds (fig. 537), arranged 

 somewhat like the leaves of a book, and offering a large surface 



Fig- 537.— King-Crab i^Lhmdus), reduced 

 On the left the under side is represented and on the 

 right an abdominal limb with its gill-folds, i, Chelicerai; 

 2-6, legs. The moutli is seen as a darkly-shaded slit 

 between the bases of the legs; 7, operculum. 



