The Arthropoda 



327 



easily noticed. Collections of this kind may be made from any waters 

 at any time of the year, from thousands of miles out at sea, and. over 

 depths of thousands of feet, to the shore line itself. The reason organ- 

 isms can be found everywhere in water is due to the fact that their whole 

 life is spent afloat, beginning with the egg and reaching through the 

 adult stage. Living organisms of this type have been called plankton, 

 and comprise protozoa, algae, diatoms, rotifera, and small Crustacea, the 

 latter being especially noticeable. 



To permit a life afloat, organisms are provided with various types 

 of adaptations, such as minute droplets of oil, long spines to add 

 buoyancy, and gelatinous envelopes. Among the small Crustacea, spines 

 and oil drops are especially abundant. Upon analysis it has been shown 

 that the oil of fish is derived from these small Crustacea. The reason 

 for this is easily understood when it is known that the sole food of 

 several species of whale and of many fish is plankton. 



TERRESTRIAL CRUSTACEANS 



The class Malacostraca ( ) are arthropoda, 



usually of large size, with five segments in the head, eight in the thorax, 

 and six in the abdomen, and a gastric mill in the stomach. These, like 

 all other classes, are divided into orders. Prominent among these orders 



Fig. 211. 



A. Ascellus aquations a 1 o 2 antennae; br, brood-pouch; k, pleopoda modified 

 to gills; md, mandibles; p x -p 7 , thoracic feet; pa^pa 6 , abdominal feet (pleopoda); 



XIV-XX, abdominal segments partly 



I-VI, head; VII-XIII, thoracic segment 

 fused. (After Hertwig.) 



B. Oniscus asellus, a terrestrial species. 



(After Paulmier.) 



are the Decapoda ( ). The crayfish comes under this 



grouping. All members of this order have the first three pairs of thoracic 

 limbs specialized as maxillipeds, and possess five pairs of thoracic walk- 

 ing-legs, while all the thoracic segments are generally covered by the 

 carapace. They also have stalked, compound eyes. 



The Isopoda ( ) have a body that is long and 



flat (Fig. 211, A), seven free thoracic segments, leaf-like legs, and no 

 carapace. There are no gills in the thorax. 



The five anterior pairs of pleopods are modified for breathing pur- 

 poses, the endopodites are thin-walled plates, and the exopodites and 

 the whole first pair of pleopods serve as a gill-cover. 



