322 ENTOMOLOGY FOR MEDICAL OFFICERS 



Order Tanaidacea. 



Minute Crustacea living, often in tubes, at the bottom of the sea. 

 The body is slender and elongate, there is a small carapace which 

 includes the head and the first 2 thoracic segments, and the second pair of 

 thoracic legs are large chelipedes. Except for the presence of a small 

 carapace they resemble Isopoda (see below). 



Order Isopoda. 



Of the members of this order the majority are marine, but some live 

 in fresh water and in subterranean waters, and one whole tribe — the wood- 

 louse tribe — is terrestrial. Of the marine species many are parasitic on 

 fishes, on other Crustacea (including Crustacea that are themselves 

 parasites), and on other marine animals. All degrees of parasitism are 

 illustrated — from predatory ectoparasites showing no sort of deterioration, 

 to sessile ectoparasites and internal parasites so utterly degenerate and 

 deformed as to have lost all resemblance to segmented animals. Among 

 some of these parasites, again, sexual anomalies are frequent : some are 

 hermaphrodite, some begin life as males and end as females, while in 

 others the male is a grub-like dwarf-parasite of the parasitic female. 

 Good instances of this last kind of involved parasitism are supplied by 

 the Bopyridce which are common parasites in the gill-chambers of 

 prawns, etc. 



In the free-living Isopoda the body is generally of a broadish pval 

 form, and is well segmented ; there is no carapace ; the thoracic 

 appendages are strong legs for crawling and clinging ; and the 

 abdominal appendages are broad plates for swimming and for respira- 

 tion. One of the deep-sea Isopods reaches the extraordinary length of 

 nearly 1 1 inches, but few others attain a quarter this size, and some are 

 quite small. Some Isopods, particularly in tropical seas, burrow into, 

 and do damage to, submerged woodwork. 



Order Amphipoda. 



Most of the Amphipoda are marine and oceanic ; some, however, such 

 as the common sandhopper — which is a good type of the order — are 



■Gmmnarus. 



littoral and amphibious ; not a few live in lakes, streams, wells, and 

 underground waters ; a few, such as the whale-lice, are external parasites, 



