JOINTED-LIMBED ANIMALS WHICH BREATHE IN WATER 405 



for a Higher Crustacean, and a pair of large rounded gills are 

 borne on the fourth and fifth of these. 



In Isopods, such as the Sea- Slaters {Idotea, &c.) and the 

 Water Wood- Louse {Asellus), the gills are delicate plates borne 

 on the inner sides of the ab- 

 dominal limbs, the outer parts ^.„„mms^^^m^^^> 

 of which protect them to some 

 extent. 



Mud- Shrimps {Nebaha, &c.) 

 breathe by means of eight pairs 

 of flattened limbs borne on the 

 thorax. These are protected by 



a large thin shield, which grows ^'^- SSS-Mud-Shrimp (JVeiaHa). enlarged 



1 1 r ^1 V 1 *-l-, ■ Left half of shield cut away. The letter o in thorax 



DaCK irOm tne neau over tniS is placed on the cleaning filament of the second jaw. 



part of the body. The second 



jaw on each side bears a flexible whip-like filament, by means ol 



which the breathing-limbs are cleaned (fig. 535). 



Many of the Lower Crustacea are of small size, and breathe 

 entirely or largely through the general surface of the body, 

 wherever the protective layer (cuticle) that covers the skin is 

 thin enough to permit of this. Special gills may, however, be 

 present in some cases, as in the larger members of the Leaf-footed 

 Crustacea (Phyllopods) such as Apus and Branchipus. These 

 possess very numerous delicate flattened limbs, each of which 

 carries a soft rounded gill. In Apus a protective shield grows 

 back from the head (as in a Mud-Shrimp) and covers a large 

 part of the body, and the inner side of this shield probably helps 

 in the work of breathing. The small Phyllopods known as Water- 

 Fleas {Daphnia, &c.) are distinguished by the shortness of the 

 body, which, except the head, is enclosed in a bivalve shell quite 

 comparable in nature to the shield of Apus. There are a few 

 pairs of flattened limbs, without gills, and these by their constant 

 movement keep up a stream of water between the halves of the 

 shell, the inner surface of which shares with them the work of 

 breathing. 



The little Mussel-Shrimps (Ostracods) breathe much in the 

 same way as Water- Fleas, but the bivalve shell is of greater 

 relative importance, and encloses head as well as trunk. The 

 limbs are narrow and specialized, and do not present the large 

 breathing surface possessed by the leaf-shaped appendages of 



