AMP1IIPODA. 



451 



organ. Appendages of the legs may also be present as additional 

 aids to copulation. The mature ova are, as a rule, carried about by 

 the female in brood pouches formed by the lamellar appendages of 

 the thoracic feet (oostegites). Development as a rule takes place 

 without metamorphosis, but the form and appendages of the young 

 animal not unfrequently differ from those of the adult animal 

 (Phronima). The segments and the appendages may even be incom- 

 plete in number after birth (Isopoda). 



Fossil Arthrostraca are found in the Oolite (Archceoniscus). Pro- 

 soponiscus occurs in the Permian, Ampliipdtis in the Devonian. 



1. Sub order. Amphipoda.* 



Arthrostraca with later ally compressed body, ivith gills on 

 thoracic feet, and an elongated abdomen, of which the three anterior 

 segments bear the 

 swimming feet, while 

 the three posterior 

 bear posteriorly di- 

 rected feet adapted for 

 springing (fig. 356). 



The Ampldpoda, 

 *are small animals, 

 being only in rare 

 cases several inches 

 long (Lysianassa 

 magellanica}. They 

 move in the water 

 principally by spring- 

 ing and by swim- 

 ming. The head, which is sometimes small (Crevettina, fig. 356), 

 sometimes large and then much swollen (Hyperina, fig. 357), is 

 sharply distinct from the thorax and is fused with the first of the 

 seven thoracic segments only in the aberrant group of the Loemodipoda. 



The two pairs of antennae usually consist of a short strong shaft 



* Besides the older works of De Geer, Kosel. M. Edwards, etc.. compare C. 

 Spence Bate, " On the Morphology of some Amphipoda of the Division Hyper- 

 ina," Ann. of Nat. Hist., Ser. 2. vol. xix., 1857. C. Spence Bate, "On the 

 nidification of Crustacea," Ann. of Nat. Hist., Ser. 3, vol. i. C. Spence Bate. 

 " Catalogue of the specimens of Amphipodous Crustacea in the collection of the 

 British Museum," London, 1862. E. van Beneden et Em Bessels, " Memoirc 

 sur la formation du Blastoderme chez les Amphipodes, etc," Bruxelles. 1868. 

 C. Claus, " Der Organismus der Phronimiden, Arlcitcn auz dem Zool. Institut. 

 dcr UnivertitW Wien, Tom II.. ]879. 



the | 

 ior ^J 



FIG. 356. Oammarus nrglectus (after G. O. Sars), with egj? 

 between the brood lamella?, (oostegites) on the thorax. 

 A', A", the two antenna? ; f, maxilliped ; JF> to F\ the 

 seven pairs of thoracic appendages ; Sf, the first swim- 

 ming foot of the abdomen. 



