190 MANUAL OF ZOOLOGY. 



ORDER II. AMPHIPODA. The members of this order resemble 

 those of the preceding in the nature of the respiratory organs, 

 which consist of membranous vesicles attached to the bases of 

 the thoracic limbs. The first thoracic segment, however, is dis- 

 tinct from the head, and the abdomen is well developed, and is 

 composed of seven segments. TJiere are seven pairs of thoracic 

 limbs, directed partly forwards, and partly backwards, the 

 name of the order being derived from this circumstance. 



Fig. 55. Amphipoda. The Sand-hopper, Talitrus locusta, enlarged. 



All the Amphipoda are small, the 'Sand-hopper' (Talitrus 

 locusta, fig. 55) and the ' fresh-water Shrimp ' (Gammarus 

 pulex) being two of the commonest forms. 



ORDER III. ISOPODA. In this order the head is always distinct 

 from the segment bearing the first pair of feet. The respiratory 

 organs are not thoracic, as in the two preceding orders, but are 

 attached to the inferior surface of the abdomen, and consist of 

 branchiae, which in the terrestrial species are protected by 

 plates which fold over them. The thorax is composed of seven 

 segments, bearing seven pairs of limbs, which, in the females, 

 have marginal plates, attached to their bases, and serving to 

 protect the ova. The number of segments in the abdomen 

 varies, but is never more than seven. The eyes are two in 

 number, formed of a collection of simple eyes, or sometimes 

 truly compound. Of the members of this order, many are 

 aquatic in their habits, and are often parasitic, but others are 

 terrestrial. 



By Milne-Edwards the Isopoda are divided into three sections, 

 termed respectively from their habits, the Natatorial, Sedentary, 

 and Cursorial Isopods. In the Natatorial Isopoda the extremity 

 of the abdomen and the last pair of abdominal legs are ex- 

 panded so as to form a swimming tail. Some of this section 

 are parasitic upon various fishes (Cymothoa), whilst others are 



