304 MANUAL OF ZOOLOGY. 



saltatorial tail. The young Amphipod acquires its full number 

 of segments and limbs before its liberation from the egg ; and, 

 as a rule, the young undergo little or no metamorphosis in 

 reaching maturity. 



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

 locusta, fig. 154) and the "fresh- water Shrimp" (Gammarus 



Fig. 154. Amphipoda. The Sand-hopper, Talitrus iocusta, enlarged. 



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. 



ORDER III. ISOPODA. In this order the head is always dis- 

 tinct from the segment bearing the first pair of feet. The respira- 

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

 but are attached to the inferior surface of the abdomen ', and con- 

 sist of branchia, 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 abdominal seg- 

 ments are coalescent, and form a broad caudal shield, beneath 

 which the branchiae are carried. The eyes are two in number, 

 formed of a collection of simple eyes, or sometimes truly com- 

 pound. The heart is sometimes an elongated tube, with 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 appendages. 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, 



