ANNULOSA: CRUSTACEA. 22$ 



All the Amphipoda are small, the "Sand-hopper" {Talitrus 

 locusta, fig. 72) and the " firesh-water Shrimp" {Gammarus pulex) 

 being two of the commonest forms. The Sand-hoppers and 

 Gammari swim on their side when in the water, and the former 

 leap with great activity on land. 



Fig. 72. — Amphipoda. The Sand-hopper, Talitrus locusta, enlarged. 



Order III. Isopoda. — In this order the head is always dis- 

 tinct 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 com- 

 posed 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. The heart is sometimes an elon- 

 gated tube, widi three pairs of fissures (as in the Amphipoda), 

 sometimes short or spherical, removed towards the abdomen, 

 and with more or fewer fissures than the above. The young 

 Isopod is developed within a larval membrane, destitute of ap- 

 pendages. After a time this membrane bursts, and liberates 

 the young, which resembles the adult in most respects, but 

 possesses only six instead of seven pairs of limbs. 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 sec- 

 tions, 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 



