152 FIRST LESSONS IN ZOOLOGY 



or swimminy feet. Has each abdominal segment a pair 

 of these ? I'^ach is composed of a basal part, tiie proto- 

 podite, and two terminal parts, an inner one, endopodite, 

 and an outer one, exopodite. In the males the first and 

 second pleopods are larger than the others. In the 

 females the pleopods serve to carry the eggs, and the 

 first two pairs are very small or absent. The last pair of 

 abdominal appendages are called uropods, and together 

 with the last segment, called the telson, form a broad, 

 flap-like tail. Is there any correspondence between the 

 uropods and pleopods ? 



Examine the appendages of the cephalothorax. They 

 may be divided into three groups, an anterior group of 

 three pairs of mouth-parts (belonging to the head) of 

 which the first pair are the mandibles and the others the 

 maxillfe ; a second group of three pairs of foot-jaws or 

 maxillipeds, belonging to the thorax; and a third group 

 of fi\'e pairs of walking-legs. The mandibles, l}'ing next 

 to the mouth-opening are hard and jaw-like, and lack the 

 exopodite ; the second maxillje have a large paddle-like 

 structure which extends back over the gills on each side 

 within the space above them called the branchial cham- 

 ber. It is b}' means of this paddle-like structure (the 

 scaphognathite) that currents of water are kept up through 

 the gill-chambers. The maxillipeds increase in size from 

 first to third pair. What pairs of walking-legs bear gills .' 

 These gills are the organs b)- which the blood is purified. 

 The blood of the crayfish flows into the large vessels on 

 the outer side of the gill, and thence into the fine vessels 

 in the leaf-like lamelku. It thus flows b}' the thin mem- 

 branous \vall of the gill on the inside; while the A\ater 

 with air dissolved in it flows b}' the thin membranous 

 wall on the outside. The ox)'gen of the air in the water 

 passes through the thin walls of the gill and blood vessels 

 into the blood, while the carbon dioxide passes from the 



