CHAPTER XXIX 



The Class Crustacea 



This Class includes the crabs, hermit-crabs, lobsters, 

 prawns and shrimps, mantis-shrimps, sandhoppers, wood-lice, 

 fish-lice, barnacles, and water-fleas, as well as a multitude of 

 small aquatic creatures not distinguished in the vernacular. 

 Though of great influence in the world's economy, as 

 scavengers of sea and shore, and as contributing a not incon- 

 siderable part of the food-supply of man, both directly in 

 themselves and indirectly as the pabulum of fishes, they are 

 not of sufficient importance, from a medical standpoint, to 

 demand much notice here. Their chief claim to our attention 

 depends upon the facts that certain small fresh-water species 

 are intermediary hosts of a parasitic worm that causes much 

 discomfort to man in certain tropical countries ; that tropical 

 land-crabs of several kinds play a minor part in the service of 

 hygiene ; and that the Crustacea that are used as food often 

 give rise to gastric irritation, having troublesome sequelae, 

 such as nettle-rash. 



In the majority of Crustacea the cuticle is strongly 

 calcified, as is implied in the name of the Class (Lat. crusta = 

 shell). 



Most of the Crustacea are aquatic and breathe by gills 

 which are attached, usually to the thoracic, but sometimes to 

 the abdominal, appendages ; but the land-crabs and wood- 

 lice breathe air direct, so long as the air is moist — the land- 

 crab by the lining membrane of the gill-chamber, or by the 

 skin of the abdomen ; the wood-lice by special air-spaces, or 

 " pseudotracheae," in the leaf-like abdominal appendages. On 

 the other hand, many small Crustacea have no special 

 breathing-organs at all. 



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