222 



ARTHROPODA 



[CH. 



Order III. Arthrostraca (Peracarida pars). 



The members of the Arthrostraca (Gr. apOpov, a joint; 

 a shell) have sessile eyes, i.e., without stalks. The carapace, which 

 in most of the above-mentioned groups covers the segments of the 

 thorax, is absent, and consequently seven of the latter are usually 



freely movable on one another, the first 

 and in rare cases the second thoracic 

 segment remaining immovably fused 

 with the head. They thus represent 

 a further stage in the same process 

 which we found going on in Cumacea 

 and Stomatopoda. Only one of the 

 thoracic appendages is modified so as 

 to form a maxillipede; there are con- 

 sequently seven pairs of walking legs 

 attached to the thorax. In the female 

 these legs bear oostegites which 

 together form a floor to the brood pouch 

 in which the eggs develop. 



The Arthrostraca are mostly small 

 animals, living in either salt or fresh 

 water ; they assume very different forms, 

 some of them having a rudimentary ab- 

 domen. They are divided into the sub- 

 orders Amphipoda (Gr. a/z<t, on both 

 ends; Tro'Sa, feet), which are for the most 

 part compressed or flattened from side 

 to side (Fig. 89) and carry their gills 

 on their thoracic appendages ; the 

 Isopoda (Gr. ros, equal ; TTOUS, TroSo's, a 

 foot), which are depressed or flattened 

 from above downwards (Figs. 90 and 91) 

 and whose gills are the modified endo- 

 podites of the appendages of the abdomen, and lastly the Tanaidacea 

 a primitive group which are only slightly flattened, in which two 

 segments are still attached to the head and there is a vestigial 

 carapace sheltering an epipodite borne by the maxillipede. A typical 

 example of the Isopoda is the Hog water-louse, Asellus aquaticus 

 (Fig. 90), common in our ponds and streams, but many of the 

 groups are parasitic and lose most of their characteristic Crustacean 

 features. 



FIG. 88. The mouth append- 

 ages of Gammarus neglectus. 

 From Leuckart andNitsche, 

 after G. 0. Sars. 



1. The left mandible. 



2. Its palp. 



3. 1st maxilla of left side. 



4. 2nd maxilla of left side. 



5. Maxillipede of each side to- 

 gether forming an under lip. 



