402 ANIMAL RESPIRATION 



The simplest way of explaining the many variations that are 

 to be found as regards number among the gills of various Higher 

 Crustaceans is to suppose that they are reductions of various 

 extent on a primitive condition, in which each of the eight rings 

 or segments making up the thorax carried four gills on each side 

 of the body, i.e. one limb-gill, two joint-gills, and one side-gill, 

 giving an original total of thirty-two. If this be the case, the 

 Lobster has lost twelve gills each side, those that remain having 

 become more efficient. This is the same principle on which the 

 gills of fishes and of molluscs have already been explained (see 



PP- 393-394)- 



The forty gills of a Lobster present a very large surface for 

 purification of the blood, but they would be of little use if there 

 were not some efficient arrangement for renewing" the water in 

 the enclosed gill-chambers. There are no cilia, as in a Lancelet 

 or Mussel, to cause a steady flow of water, and other means have 

 therefore to be employed. The movements of the limbs, to which 

 limb-gills and joint-gills are attached, must do something in this 

 direction, and the more quickly a Lobster moves about the better 

 they act. They are not able, however, to do more than stir up 

 the water, without producing a steady current in a definite direc- 

 tion. Such a current does exist, the impure water flowing out 

 steadily from the front of each gill-cavity, and careful observation 

 of an aquarium specimen shows how this is brought about. Just 

 at the front of each gill-chamber something will be seen in con- 

 stant movement, and examination of a dissected Lobster shows 

 that this something is the edge of a boat-shaped plate, the baler, 

 which lies within the gill-chamber in front of the gills (fig. 532), 

 and constantly scoops out the water, hence its name. The plate 

 is part of the last of the three jaws belonging to the head (second 

 maxilla), and during life these jaws are in constant movement from 

 side to side, which of course keeps their baling plates workincr. 

 Another very interesting device also deserves notice. It is 

 extremely important for parasites and particles of dirt to be kept 

 out of the gill-chamber, and this is managed to some extent by 

 tufts of long hair-like bristles which are attached to the bases of 

 some of the limbs, and strain the water as it enters the o-iH- 

 chamber. 



The Freshwater Crayfish [Astacus fl^iviatilis) has undergone 

 greater reduction than the Lobster as regards its breathing oroans, 



