AMPiiiroiiA. 3 



hut niufh loss dovolopcd than the precetUng, aiul their liasji ('J) 

 s(juainifonu and hirgely developed ; these are always directed back- 

 wards. In the Aberkantia the coxiB (1 ) are fused with the segments 

 of the pereion to which they belong. These five pairs are the am- 

 biilatoiy organs of the animal, and efficientlj' fulfil their design. The 

 branchiae, consisting of a single saccular vesicle, arc pendent from 

 the coxa (1) of each limb, except the fia-stgnathopoda, and sometimes 

 the posterior pereiopoda, in the males. An incubatory pouch is 

 formed by a single foliaceous plate, ciliated at the margins, being 

 given off from the coxa? of the four anterior Kmbs in the females ; in 

 this the ova are deposited, and the young developed, luitU they have 

 arrived at a period when they difter from the parent in but a Umited 

 degree ; except in the Hyperina, where the form of the general con- 

 tour of the laiTa dift'ers much from that of the parent. 



The pleopoda, six in number, are developed upon two types. The 

 three anterior pairs (p, q, ?') are placed within the lateral margins; 

 they consist of a uniarticulate peduncle, and two branches of multi- 

 articulate flexible fiageUa, bearing a pair of jjlumose ciha on eveiy 

 articulation. TTie three succeeding paii's (s, t, u) are placed upon 

 the latero-infeiior margins, and consist of a uniavticulate peduncle 

 that supports two branches, inflexible, generally luiiarticixlate, and 

 styliform, fringed with strong hairs or spines. The posterior varj- 

 in form, and are a valuable assistant in the determination of species. 

 In some of the Hyperixa they assume a foliaceous character. 



The anterior pleopoda (p, q, r) are the organs by which the 

 animal swims about, which it does both gracefully and actively. 

 The posterior (s, t, u) enable it to dart from place to place mth con- 

 siderable energy : by folding the pleon close beneath the body and 

 suddenly striking it out again, it springs to a considerable distance. 

 This capability is wanting in the Aberrakttia, where the pleopoda 

 are all either absent or nichmcntai-y. 



The teLson (r) is the i-udiment of the last segment of the animal ; 

 it is insignificant in its appearance, but a valuable aid in the defi- 

 nition of genera, in consequence of the variety of forms that it 

 undergoes ; its typical appearance is that of an acute-angled triangle, 

 the apex being rounded off, or cleft. 



Tlie average length of the Amphipoda is less than an inch. The 

 largest specimen is about two inches and a half, the smallest about 

 one-tenth of an inch. 



To compensate for their minute size, their importance is raised by 

 theii- number. This increases towards the Arctic regions, where 

 they are appropriated as food by the whales and other animals. 



The Order, as here desciibed, is sjTionymous with the third tribe 

 (Amphipoda) of Dana's order of Cuoristopoua, and embraces the two 

 orders of AjrpuiPODA and L.^modipoda in Mibie-Edwards's arrange- 

 ment. 



The AifpniPonA naturally separate into two groups. — the one 

 being constant to the iioniial type of the Ordei-, the other departing 

 in many of its chai-acteristics. We therefore arrange them respect- 

 ivelv under the heads of Noumm.tv and Abehrantia. 



B 2 



