CRUSTACEA IN GENERAL 143 



dance of mackerel has recently been studied with the following 

 results : The number of fish depends upon the number of Crus- 

 tacea that are available for food. These Crustacea feed upon 

 minute plants, mostly diatomes, that float about near the sur- 

 face of the sea, and their abundance must depend upon the 

 abundance of these plants. The plants require sunlight for their 

 growth and multiplication, so that the amount of sunlight con- 

 trols the number of plants. Actual observations have shown 

 that a season of bright sunshine is followed by good fishing, and 

 a cloudy one always results in a poor catch of mackerel. The 

 relations here indicated remind one of those pointed out by Dar- 

 win between bees and clover (see p. 4). 



Injuries Due to Crustacea. - - Very few Crustacea are injurious 

 to man. The damage done by the crayfish has already been 

 noted (p. 135). Several species make burrows in wood and 

 often do considerable damage to the timbers in piers. Wood 

 that is placed in situations open to attack by little Crustacea is 

 commonly treated with creosote. 



One species, Cyclops, is the means of transmitting the para- 

 sitic guinea worm which causes the appearance of dangerous 

 abscesses on the legs of people living in tropical Africa. The 

 young worms that chance to fall into water penetrate the body 

 of the Cyclops where they live. Sometimes a parasitized crus- 

 tacean is swallowed by man, as may easily happen in drinking 

 water from a pond. In the alimentary canal of man the worms 

 are freed, after which they bore their way through his body until 

 they reach the legs, where they produce the abscesses. 



Characteristics and Classification. - - The Crustacea are ar- 

 thropods most of which live in the water and breathe by means 

 of gills. The body is divided into head, thorax, and abdomen, 

 or the head and thorax may be fused, forming a cephalothorax. 

 The head usually consists of five segments fused together; it 

 bears two pairs of antennae (feelers) , one pair of mandibles (jaws), 

 and two pairs of maxillae. The thorax bears a variable number 

 of appendages, some of which are usually locomotory. The ab- 



