170 



INTRODUCTION TO ZOOLOGY 



The abdomen consists of six segments bearing smaller hairy 

 appendages, and, at the end of the body, a small single tail- 

 piece ov telson, as in shrimps and prawns. The name Am- 

 phipoda given to this order of Crustacea, refers to the two 

 types of ambulatory limbs which are conspicuous on the 

 body, the anterior pairs projecting forwards, whilst the hind 

 pairs are much longer and turn backwards. 



Gammarus swims very actively, either by the movements 

 of the abdomen, or by jerking itself along with these three last 

 pairs of thoracic legs. The three anterior pairs of abdominal 

 legs are always in motion, driving a current of water over 

 the gills. 



The male is considerably bigger than the female, and is 

 often to be seen carrying the female in front of him, holding 

 her by the anterior thoracic legs. 



Sand-hoppers and Shore-hoppers are very similar to the 

 fresh-water shrimps, and also are invaluable scavengers ; they 

 are common everywhere on our coasts, burrowing in the sand 

 above high-water mark. They, too, have thoracic and abdo- 

 minal appendages of different types, and therefore are in- 

 cluded in the order Am- 

 phipoda. 



Order 3 : ISOPODA 



Water TheCom- 



Slatera and mon Water 

 Wood-lice. Slater, or 

 Water Louse (Asellus 

 aquaticus), is, like Gam- 

 marus, a small scavenger 

 very common in pond 

 and river (Fig. 106). 

 It differs, however, in 

 its body being flattened 

 dorso-ventrally instead 

 of laterally, and in all 

 its legs being of the 

 same type (hence the name of the order, Isopoda). Also the 

 gills are attached to the abdominal appendages instead of to 



Fig. 106. — The Water Slater {Asellus 



aquaticus). 



(Actual length is shown by the line to the left.) 



