CHAPTER XXXIV 



ANIMAL RESPIRATION JOINTED-LIMBED ANIMALS 

 WHICH BREATHE IN WATER 



Two classes of Arthropods exemplify adaptations for breathing* 

 in water, i.e. Crustaceans and K ing-Crabs. And as in other 

 groups of animals, the higher members are more specialized in 

 regard to their breathing organs, as well as in other respects. 



CRUSTACEANS (CRUSTACEA) 



The HIGHER CRUSTACEA, such as Lobsters, Crayfish, Prawns,. 

 Shrimps, and Crabs, possess large and complex gills, all of which 

 are borne on the thorax, or region which succeeds the head. The 

 Common Lobster (Homarus vulgaris] is a convenient first example, 

 and it may be premised that the eight pairs of limbs borne by its 

 thorax are, beginning in front, three pairs of foot-jaws which help 

 to tear up the food, the huge pincers, and four pairs of walking- 

 legs (see vol. i, pp. 403-405). On first inspection no gills at 

 all are visible, and this is because a special shelter has been pro- 

 vided for these delicate structures. There is, in fact, a spacious 

 gill-chamber on each side of the body, covered by a large gill-cover 

 extending down to the bases of the limbs. There is, however, a 

 slit by which water can enter behind and below, and go out in 

 front. On removing this protective covering (which recalls the 

 gill-cover of a bony fish as regards its use) a number of large 

 gills are seen (fig. 532). Each of them is like a bottle-brush (to 

 use Huxley's simile), and consists of a central axis beset with 

 numerous threads. On the outside are six limb-gills, attached to 

 the bases of the last two pairs of foot-jaws, the pincers, and the 

 first three pairs of walking-legs. On turning these back ten joint- 

 gills are seen, attached in pairs to the junctions between the limbs 

 and body from the last foot-jaws to the third pair of walking-legs,. 



400 



