JOINTED-LIMBED ANIMALS WHICH BREATHE IN WATER 403 



for it possesses only eighteen gills each side, there being, for 

 example, only one side-gill, the last of its series. But in front 

 of this are two little threads placed where other side -gills are 

 situated in the Lobster, and these are no doubt vestiges of such 

 gills, which Crayfishes once possessed but have now all but lost. 



A very large reduction has taken place in the Common Prawn 

 {Palcemon serratus), for it has but eight gills each side, i.e. one limb- 

 gill, two joint-gills, and five side-gills. In spite of the reduction 

 it will be seen that it possesses one more of the last kind than the 

 Lobster. It is also to be noted that the Prawn's grills, instead of 

 being like bottle-brushes, are plume-shaped, consisting of a central 

 axis bearing two sets of leaflets, much as in the typical gill of a 

 Sea-Snail (see fig. 523, p. 394). 



Some of the Prawns and Shrimps use the first walking-leg for 

 cleaning the gills, pushing it into the front end of the gill-chamber 

 for this purpose. 



The Common Hermit-Crab [Pagurus Bernhardus) cleans its 

 gills by means of the last pair of walking-legs, which are small, 

 and, like the pair in front of them, find their chief use in holding 

 on to the shell in which their owner lives. 



Crabs constitute the most highly specialized of all the groups 

 of Higher Crustaceans, and exhibit a large amount of reduction 

 and modification in their breathing organs. The most typical 

 species, of which we may take the Edible Crab {Cancer paguTits) 

 as a type, are shore-forms, which spend a good part of their time 

 out of water, so that special provision is necessary to prevent 

 their gills from drying up by exposure to the air. On examining 

 the type mentioned we shall find that the gill -cover is in such 

 close contact with the bases of the limbs that water cannot enter 

 the gill-chamber from behind or below, as in the Lobster and 

 its allies. There is a special aperture in front for this purpose, 

 which can be closed when necessary by a sort of door or valve 

 constituted by a part of the last pair of foot-jaws. Waste water 

 is scooped out by the baler through two other openings situated 

 still farther to the front. On opening one of the gill-chambers 

 seven large plume-gills are at once visible, of which five are joint- 

 gills, while the other two are equivalent to the first two side-gills 

 of a Lobster. There are also two limb-gills belonging respectively 

 to the second and third foot-jaws. There is no gill attached to 

 the first foot-jaw, but it possesses a large plate or epipod (belong- 



